Nuclear waste storage canisters, welds, and method of fabricating the same

ABSTRACT

A dry storage systems for radioactive nuclear waste materials may include a double-walled canister system. The canister system may include a canister having a tubular inner shell defining an internal cavity for storing nuclear waste material, a first lid sealably welded to a first end of the inner shell, a primary base plate defining a peripheral edge portion and having an annular closure flange, and an annular full thickness butt weld formed at an abutment joint between the annular closure flange and a second end of the inner shell. The inner shell, first lid, and first end closure may collectively define a sealed primary pressure retention barrier. A tubular outer shell may adjoin the inner shell. The outer shell may be welded to the canister to form a hermetically sealed secondary pressure retention barrier.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 16/695,450, filed Nov. 26, 2019, which claims the benefit U.S.Provisional Patent Application Ser. No. 62/772,986 filed Nov. 29, 2018.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 15/294,896, filed Oct. 17, 2016, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 62/242,458,filed Oct. 16, 2015.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 15/709,098, filed Sep. 19, 2017, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 62/396,565,filed Sep. 19, 2016.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 16/116,994, filed Aug. 30, 2018, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 62/551,914,filed Aug. 30, 2017.

The entirety of each of the aforementioned applications is incorporatedherein by reference.

BACKGROUND

In the operation of nuclear reactors, the nuclear energy source is inthe form of hollow zircaloy tubes filled with enriched uranium,collectively arranged in multiple assemblages referred to as fuelassemblies. When the energy in the fuel assembly has been depleted to acertain predetermined level, the used or “spent” nuclear fuel (SNF)assemblies are removed from the nuclear reactor. It is necessary toremove these fuel assemblies from the reactor after their energy hasbeen depleted to a predetermined level. Upon depletion and subsequentremoval from the reactor, these spent nuclear fuel (“SNF”) rods arestill highly radioactive and produce considerable heat, requiring thatgreat care be taken in their subsequent packaging, transporting, andstoring. Specifically, the SNF emits extremely dangerous neutrons andgamma photons. It is imperative that these neutrons and gamma photons becontained at all times subsequent to removal from the reactor core.

The standard structure used to package used or spent fuel assembliesdischarged from light water reactors for off-site shipment or on-sitedry storage is known as the fuel basket. The fuel basket is essentiallyan assemblage of prismatic storage cells each of which is sized to storeone fuel assembly that comprises a plurality of individual spent nuclearfuel rods. The fuel basket is arranged inside a cylindrical metallicstorage canister (typically stainless steel), which is often referred toas a multi-purpose canister (MPC), which forms the primary containment.The canister is then placed into an outer ventilated overpack or cask,which forms the secondary containment, for safe transport and storage ofthe multiple spent fuel assemblies. The ventilation utilizes ambientcooling air to dissipate the considerable heat still emitted by thespent fuel.

The used or spent nuclear fuel contained in the fuel basket inside thefuel canister is stored in an inert gas atmosphere formed within thecanister. Guaranteed sequestration of heat and radiation emitting usednuclear fuel from the environment under all storage or transportconditions is an essential design requirement for the canister. Thisassurance of confinement requirement has been fulfilled in the presentstate-of-the-art by hermetically seal welding the top lid to thecanister shell after the spent fuel has been loaded into the canister(typically under water such as in the spent fuel pool of a nuclearreactor). The all-welded canister provides guaranteed confinement of thecontents, but makes the stored fuel difficult-to-access if repackagingis required at a later date. While lid cutting tools to sever the lidfrom the canister shell have been successfully developed anddemonstrated, the cutting operation is inherently dose-accretive,cumbersome, and time-consuming requiring metal chip and lubricantmanagement during the process.

In defueling a nuclear reactor, the SNF may be removed from the reactorand placed under water, in what is generally known as a spent fuel poolor pond storage. The pool water facilitates cooling of the SNF andprovides adequate radiation shielding. The SNF is stored in the pool fora period of time that allows the heat and radiation to decay to asufficiently low level so that the SNF can be transported with safety.However, because of safety, space, and economic concerns, use of thepool alone is not satisfactory where the SNF needs to be stored for anyconsiderable length of time. Thus, when long-term storage of SNF isrequired, it is standard practice in the nuclear industry to store theSNF in a dry state subsequent to a brief storage period in the spentfuel pool. Dry storage of SNF typically comprises storing the SNF in adry inert gas atmosphere encased within a structure that providesadequate radiation shielding.

Maintaining confinement of radioactive material contents such as SNFs orother contaminated waste materials is a principal function of weldedcanisters used heretofore. One such canister, known as a multi-purposecanister (“MPC”), is described in U.S. Pat. No. 5,898,747, to Krishna P.Singh, issued Apr. 27, 1999, the entirety of which is herebyincorporated by reference. Typically, the SNF is loaded into an opencanister that is submerged under water in a fuel pool. Once loaded withSNF, the canister is removed from the pool, placed in a staging area,dewatered, evacuated, dried, hermetically sealed, and transported to along-term storage facility. The canister typically contains an internalbasket comprising an array of cells for storing and supporting aplurality of SNF rods positioned within the cavity of the canister (see,e.g. U.S. Pat. No. 5,898,747).

In its standard form, the confinement boundary of the canister consistsof a thick base plate welded to a relatively thin walled cylindricalshell. A thick walled top lid is typically welded to the top extremityof the shell after the used fuel is loaded in the canister leading to anall-welded confinement boundary. Thus, the so-called “high level waste,”which includes used nuclear fuel, is confined in a space enclosed by acylindrical canister consisting of a relatively thin wall shell weldedto a thick baseplate and a thick top lid welded to its lower and upperextremity, respectively (see, e.g. FIG. 26A). The bottom and topenclosures are plate-type structures that must be sufficiently thick towithstand the design pressure applicable to the canister's role as apressure vessel for forming an appropriate pressure retention boundary.The base plate and the top lid are typically in the range of about 3 to9 inches thick, respectively (the top lid is usually much thicker thanthe base plate to limit the radiation dose to the lid welding crew).Sometimes, a narrow annular top closure ring is welded to the canisterlid to protect vent and drain ports, as illustrated for example in FIG.26B.

The welded canisters are generally stored inside ventilated outermodules called “casks” or “overpacks” which come in two known storagesystem variations: horizontal storage systems in which the canisters arestored horizontally (see, e.g. FIG. 27A) and vertical storage systems inwhich the canisters are stored vertically (see, e.g. FIG. 27B). Thematerial of construction that is almost universally used in the nuclearwaste material storage inner canister is classically austeniticstainless steel or one of its more recent adaptations known as Duplexstainless steel. Stainless steel is remarkably resistant to all types ofcorrosion except stress corrosion cracking (SCC), whose occurrencerequires three parameters to be simultaneously present, namely a tensilestress field in the surface exposed to the environment, an adequateconcentration of halides, and all accompanied by a high relativehumidity in the ambient air. An adequate halide concentration level istypically present at storage facilities located at seasides which helptrigger SCC. The threshold humidity level, typically considered to be20%, is also present at coastal sites. Because the used fuel packaged ina canister is heat emitting (as much as 50 kilowatts, in some cases),most of the surface of the canister is heated which results in aconcomitant reduction in the humidity of the air in contact with theexternal surface of the canister (as the air heats up, its relativehumidity decreases). Thus, the portion of the canister surfacevulnerable to SCC is the shell (the top and bottom plates are thick andtherefore have a far larger reserve against thru-wall crackpropagation). More specifically, the most vulnerable portion of theshell is the region that is not sufficiently hot and hence in contactwith humid air. In vertical canisters, the bottom region or portionwhere the coldest air hits the canister in the ventilated cask/overpacksis most vulnerable to SCC.

As storage of the nuclear fuel in welded canisters for extended periodshas increasingly become the most preferred approach in the U.S., UK,Spain and many other countries, the need to assure a long service lifehas become important. Peening the vulnerable surfaces to introduce acompressive stress state appears to be an effective means to eliminateone of the trio of SCC (tensile stress) inducing parameters. However,the results of this measure will not be known for a long time.

Improvements in the traditional spent nuclear fuel canisters whichovercomes the foregoing deficiencies are desired.

The canister used to store hazardous materials such as used or spentnuclear fuel (SNF) is typically made from multiple courses of shellsegments butt welded to each other leading to discrete linear seams orjoints, thereby forming circumferential joints as shown. In addition,each shell segment in turn may also be formed of a cylindrically rolledplate or sheet of material which is butt welded at the adjoining ends oredges forming longitudinal joints. The typical material of constructionused to store high level nuclear waste is austenitic stainless steel.Stainless is extremely ductile, has a high fracture resistance at evencryogenic temperatures and it resists corrosion in a wide variety ofenvironments. These properties of stainless steel have made it a prizedmaterial for making components that may be exposed to adverseenvironmental conditions in open air settings. In the nuclear powerindustry, the canisters used to store used nuclear fuel (often calledthe multi-purpose canister or “MPC”) are almost always made from astainless alloy. While the wide usage of austenitic stainless steel isgrounded on its solid track record, a stainless weldment has one notablevulnerability: the material is susceptible to stress corrosion cracking(SCC) if its exposed surface has a tensile stress field and is subjectedto a prolonged exposure to a salt-laden atmosphere such as a chloride orhalide-bearing moist marine air of certain relative humidity. It isrecognized that residual tensile stresses created in the weld andadjoining heat affected zones (HAZ) by welding the canister shell are asource of such stress fields. A high level of tensile stress on theexposed surface, humid air, and a salt species must be all present toinitiate stress corrosion cracking.

Prior experience shows that a stainless shell of an MPC withstands humidmarine air and have remained intact (without any breach) for decades inservice which provides comforting assurance to the canister users in thenuclear industry. There is general consensus in the scientific communitythat there is no credible threat to the integrity of the canistersstored even in salt air environments for several decades. However, thepotential for eventual degradation has been sufficient to prompt theefforts to extend the canister's service life. Towards this end, severalorganizations in the nuclear industry have launched programs forcomprehensive “Aging Management” of MPCs.

A need exists for an improved welded nuclear waste storage canisterwhich can prolong the onset of SCC.

Manufacturing a welded cylindrical shell-type structure or weldmentrequires rolling or forming of metal plate stock followed by welding toclose the seams or joints. The multi-purpose canister (MPC) is anexample of a cylindrical shell-type structure or weldment used in thenuclear power generation industry. Such canisters, used to store spentnuclear fuel (SNF) and other forms of nuclear high-level waste (HLW),are typically made of austenitic stainless steel. Stainless is anexcellent material for this application for a variety of reasons such ashigh ductility and excellent fracture resistance in the entire range oftemperatures for which the canisters are designed (−40 deg. C. to 400deg. C.).

Austenitic stainless, however, has one drawback: while otherwiseextremely resistant to general corrosion effects, it is somewhatvulnerable to stress corrosion cracking (SCC) under certain set ofenvironmental conditions. The conditions necessary to induce SCC in thestainless-steel canister exposed to the ambient environment are: (1) Astate of tensile surface stress on the surface exposed to theenvironment; (2) Presence of halides in the ambient environment; and (3)A state of high relative humidity in the ambient air.

Canisters stored inside an outer ventilated module or overpack in a saltwater environment may fulfill all of the above conditions most of thetime, thereby making them vulnerable to the onset of SCC. The region ofthe canister most susceptible to SCC is where the state of surfacestress in the shell-type structure is invariably tensile. Such tensileregions are the weld seams and possibly the adjacent heat affected zonesin the shell base material adjoining the welds where residual tensilestress is caused by the shrinkage of the weld puddle and thermaltransient effects.

Such rolled cylindrical shell-type structures are further characterizedby residual stresses from rolling as well as welding operations, furtherexacerbating the SCC problem. Rolling produces a radially symmetricstress field which is compressive on the outside surface and tensile onthe inside. This is a favorable situation for the shell to prevent toSCC because the surface exposed to the ambient environment with thecompressive stress field is the outer surface. FIG. 45 shows the typicalresidual stress distribution in the circumferential direction in theshell subsequent to rolling the originally flat plate into a cylinder.

Unlike the residual stress field from plate rolling, the residualtensile stress produced by welding of the seams or joints is locallyconcentrated reaching its peak in the center of the weld line andattenuating gradually away from it. The weld along the weld line isusually in a tensile state on the outer surface of the weld exposed tothe ambient environment, which unfortunately makes it the prime locationfor SSC attack.

An improved method or process to form and protect shell-type weldmentsfrom SCC, such as particularly those used to create the multi-purposecanister (MPC) for storing spent nuclear fuel discussed above, isneeded.

BRIEF SUMMARY

To overcome the foregoing limitations in the art for retrieving thespent nuclear fuel (SNF) contents from “all-welded” fuel canisterconstructions presently used in the nuclear industry, a new and improvedspent nuclear fuel canister is disclosed herein which not only maintainsthe essential features of the canister's structural ruggedness forprotecting the fuel, but also makes the fuel more readily accessiblewithout the foregoing cutting process, and with minimum human effort andradiation exposure to the workers. Some embodiments further include heatdissipation features for significantly increasing the heat rejectioncapability of the canisters, thereby safeguarding the structuralintegrity of the SNF stored therein. Also importantly, the SNF canistersdisclosed herein advantageously maintain the same preferred smalldimensions and profile (i.e. height and diameter) of prior canisterswith seal welded lids, thereby allowing the new canisters to be usedinterchangeably in existing outer transport and storage overpacks orcasks without modification.

The SNF canister according to the present disclosure includes amulti-thickness shell and compact bolted closure lid-to-shell joint forready access to the fuel contents inside. This eliminates thetime-consuming and cumbersome prior cutting processes described abovewhich are required to sever a welded joint between the lid and shell inwelded lid designs. In one embodiment, the present lid may be directlybolted to the top of the shell.

To accommodate the bolting and seals required, a multi-thickness shellis provided having a top fastening portion that comprises areinforcement structure in the form of an annular mounting bossintegrally formed with the shell. The top fastening portion of the shellhas a greater transverse wall thickness than the wall portion of theshell below, thereby providing additional purchase for engaging thebolts at the bolted lid joint. In some embodiments, the mounting bossmay have a wall thickness equal to or greater than at least twice thethickness of the lower shell wall.

In various embodiments described herein, the upper annular mounting bossmay protrude radially inwards into the cavity of the shell beyond itslower inner surface, or alternatively protrude radially outwards beyondthe lower outer surface of the shell. The boss or fastening portion ofthe shell comprises a plurality circumferentially spaced and upwardlyopen threaded bores formed in the top of the shell at the fasteningportion. The bores threadably engage the bolts which extendlongitudinally through the lid. An inner and outer seal are provided toseal the containment cavity of the SNF canister and provide redundanthigh integrity leak barriers.

In some preferred embodiments, the top mounting boss/fastening portionmay be formed as a monolithic unitary structural portion of the shellwhich may be one piece. In other embodiments, the mountingboss/fastening portion may be a discrete element seal welded to thelower smaller thickness portion of the shell.

The closure lid has an annular mounting flange receiving the throughbolts. The flange is seated on the top end of canister shell.Significantly, the mounting flange does not protrude radially beyond theouter surface of the either the upper fastening portion or lowerportions shell to minimize the outside diameter of the canisternecessary for storing the canister inside the an outer radiationshielded overpack or cask for transport/storage. This unique lid andbolting construction and arrangement advantageously results in a compactlid design, thereby keeping the outer cask's outside diameter to thesmallest possible which is an essential part of a design that complieswith the NRC's 10CFR71 regulations. Although bolted lids may be used inthe bulker radiation shielded outer transport/storage casks, suchbulkier designs are not suit for the inner SNF canister which mustmaintain the smallest outer diameter and profile possible withoutsubstantially reducing the number of spent fuel assemblies which bestorage inside the canister.

In one embodiment, the canister may further comprise a plurality ofradial cooling fins arranged perimetrically on the outer surface of theshell to enhance heat dissipation. The fins may be welded directly tothe outer surface of the shell or may be integrally formed therewith toprovide direct contact. This ensures an effective conductive heattransfer path from the shell to the outer environment surrounding thecanister, thereby allowing the fins to act as heat radiators. In someconstructions, the fins may be disposed in an annular 360 degreerecessed lower area of the outer shell formed by the mounting boss. Bylocating the fins in the recessed area below the mounting boss, the finsadvantageously do not protrude radially outwards beyond the lid, shell,and bottom baseplate of the canister in some implementations to maintainthe desired small outside diameter of the canister package, andimportantly to protect the fins from damage when handling and moving thecanister during the spent fuel dewaters, staging, and transportoperations.

In one aspect, a canister for spent nuclear fuel storage comprises: alongitudinal axis; an elongated shell extending along the longitudinalaxis, the shell including a top end and a bottom end; a cavity extendingalong the longitudinal axis inside the shell for storing spent nuclearfuel; a baseplate attached to the bottom end of shell and enclosing alower portion of the cavity; a closure lid detachably fastened to thetop end of the shell and enclosing an upper portion of the cavity; and aplurality of mounting bolts extending longitudinally through the lid andthreadably engaging the top end of the shell; wherein the canister isconfigured for placement inside an outer overpack with radiationshielding.

In another aspect, a canister for spent nuclear fuel storage comprises:a vertical longitudinal axis; a cylindrical shell extending along thelongitudinal axis, the shell including a top end, a bottom end, and anouter surface; an internal cavity extending between the top end andbottom end of the shell along the longitudinal axis for storing spentnuclear fuel; a baseplate attached to the bottom end of the shell andenclosing a lower portion of the cavity; a closure lid detachablyfastened to the top end of the shell and enclosing an upper portion ofthe cavity, the lid having a circular body comprising a first portionand a second mounting flange portion protruding radially outwards beyondthe first portion; and a plurality of mounting bolts extendinglongitudinally through the mounting portion of the lid and threadablyengaging the top end of the shell; wherein the mounting flange portionof the lid does not protrude radially outwards beyond the outer surfaceof the shell; wherein the canister is configured for placement inside anouter overpack with radiation shielding.

In another aspect, a canister for spent nuclear fuel storage comprises:a vertical longitudinal axis; a cylindrical shell extending along thelongitudinal axis, the shell including a top end and a bottom end; acavity extending along the longitudinal axis inside the shell forstoring spent nuclear fuel; a baseplate attached to the bottom end ofshell and enclosing a lower portion of the cavity; a closure liddetachably fastened to the top end of the shell and enclosing an upperportion of the cavity; and a plurality of mounting bolts extendinglongitudinally through the lid and threadably engaging the top end ofthe shell; and a plurality of longitudinally-extending cooling finsprotruding radially outwards from the shell, the fins spacedperimetrically apart around the shell; wherein an outer surface of thelid is substantially flush with an outer surface of the top end of theshell; wherein the canister is configured for placement inside an outeroverpack with radiation shielding.

A system for storing spent nuclear fuel comprises: a longitudinal axis;an elongated outer cask comprising a double-walled first shell includinga radiation shielding material, a first lid attached to a top end of thefirst shell, and an internal first cavity; an elongated inner cylindercanister positioned in the first cavity of the first shell, the cylindercomprising: a single-walled second shell extending along thelongitudinal axis, the second shell including a top end and a bottomend; a second cavity extending along the longitudinal axis inside thesecond shell, the second cavity containing spent nuclear fuel; abaseplate attached to the bottom end of shell and enclosing a lowerportion of the second cavity; a second lid detachably fastened to thetop end of the second shell and enclosing an upper portion of the secondcavity; and a plurality of mounting bolts extending longitudinallythrough the second lid and threadably engaging a plurality of blindthreaded bores formed the top end of the second shell; the threadedbores formed in a radially projecting mounting boss extendingcircumferentially around the top end of the second shell, the mountingboss having a greater transverse first wall thickness than a transversesecond wall thickness of lower portions of the second shell below themounting boss.

In one aspect, a canister for dry storage of nuclear waste materialcomprises: a tubular first shell defining an internal cavity for storingnuclear waste material; a lid sealably attached to a first end of thefirst shell; an end closure attached to a second end of the first shell,the first end closure comprising a base plate and an upturned annularclosure flange disposed on a peripheral portion of the base plate; acircumferentially-extending butt joint formed between the annularclosure flange and the second end of first shell; and acircumferentially-extending butt weld formed at the butt joint whichhermetically seals the annular closure flange to the second end of thefirst shell.

In another aspect, a double-walled canister system for dry storage ofnuclear waste material comprises a canister including: a tubular innershell defining an internal cavity for storing nuclear waste material andhaving a first height; a first lid sealably welded to a first end of theinner shell; a primary base plate defining a peripheral edge portion andhaving an upturned annular closure flange disposed on the peripheraledge portion; and an annular full thickness butt weld formed at anabutment joint between the annular closure flange and second end of theinner shell which sealably attaches the first end closure to the innershell. The inner shell, first lid, and first end closure collectivelydefine a hermetically sealed primary pressure retention barrier. Atubular outer shell adjoins the inner shell, the outer shell having asecond height which is at least coextensive with the first height of theinner shell; wherein the outer shell is welded to the canister to form ahermetically sealed secondary pressure retention barrier.

A method for fabricating a canister system for dry storage of nuclearwaste materials is provided. The method comprises: providing a tubularfirst shell having a first end, a second end, and cavity extendingbetween the first and second ends for storing nuclear waste materials,and a first base plate having an upturned annular flange defining acircumferential edge; abutting the circumferential edge of the annularflange against the first end of the first shell to form a butt joint;welding the circumferential edge to the first end of the first shellwith a full thickness butt weld to form a hermetic seal; and welding anannular peripheral edge of a first lid to the second end of the firstshell to form a hermetic seal.

In certain parts of the present disclosure, certain preemptive measureseffective against stress corrosion cracking (SCC) in the MPC arepresented that can be implemented during the nuclear waste canister'sfabrication to forestall the incidence of SCC after prolonged exposurein an adverse ambient environment for many more decades. The firstmeasure generally comprises using a welding process which minimizes thequantity of weld metal for making a thru-thickness but weld, such as forexample by hybrid laser welding. If a double bevel conventional weldingtechnique such as MIG or submerged arc welding must be used, then theweld is preferably made from the outside first to minimize the size ofthe outer exposed bevel and thus amount of filler wire. By making theoutside weld first, the weld can shrink freely thus minimizing residualtensile stress imparted to the HAZ of the shell by welding. A directresult of the reduced weld mass is reduction in the magnitude and extentof the tensile stress particularly on the outer exposed surface of theweld and the HAZ which forms a potential initiation site for SCC.

The second measure generally comprises forcibly compacting andcompressing the exterior convex weld crown to plastically deform andflatten it causing a through-thickness compaction (TTC) and state ofcompressive stress in the HAZ. The size of the crown to yield bestresults may preferably be determined by a finite element analysis usingthe actual geometry of the weld, material properties of the base andweld materials, and thickness of the canister shell parts being joined.Preferably, the foregoing two measure or techniques can be used togetherfor best results. Optionally, surface peening may be appliedsubsequently to weld and HAZ after TTC to further reinforce the state ofcompressive stress in the weld region.

A method or process for fabricating a nuclear waste canister weldmentformed from stainless steel plate or sheet susceptible to SCC maytherefore generally include in one embodiment forming a weld at a seamor joint in the sheet or sheets, and compressively compacting andflattening the weld with sufficient force to convert the residual stressfield in the weld and HAZ resulting from welding to a compressive stressfield for a full depth or thickness of the shell base material. The weldpreferably may be formed of a type and with profile characteristicswhich reduce tensile stresses initially created in the weld and HAZ byheat produced during the welding operation, thereby mitigating orpostponing the occurrence of stress corrosion cracking (SCC) in thesevulnerable zones. As noted above, the compaction and flattening stepconvert the tensile stresses created in the weld and HAZ material of theshell base material to a full depth or thickness of the weld and basematerial. The peening step may optionally be used after TTC to impart afurther degree of compressive stresses in the weld and HAZ at the outersurface region of the canister shell (i.e. less than full depth),thereby forming an additional shield against SCC.

In one aspect, a method for fabricating a nuclear waste canistercomprises: providing a stainless steel sheet or sheets for a nuclearwaste canister, the sheet or sheets including an exterior surface, aninterior surface, and an open joint defined between adjacent edges ofthe sheet or sheets; forming a full thickness weld in the open joint,the weld extending from the interior surface of the sheet or sheets to aconvexly rounded crown at the exterior surface of the sheet or sheets;compressing the weld for a full thickness of the weld; and flatteningthe crown of the weld such that the crown after compressing issubstantially flush with the exterior surface of the sheet or sheetsadjoining the weld.

In another aspect, a method for fabricating a nuclear waste canistercomprises: providing a stainless steel shell or shells for a nuclearwaste canister, the shells or shells including an exterior surface, aninterior surface, a thickness defined therebetween, a pair of spatiallyseparated opposing edges to be joined defining an open weld joint; andforming a full thickness weld in the weld joint by first forming anexterior weld mass at an exterior of the weld joint followed by formingan interior weld mass at an interior of the weld joint, the weldextending from the interior surface to the exterior surface of the shellor shells.

In another aspect, a method for fabricating a nuclear waste canistercomprises: providing a stainless steel shell or shells for a nuclearwaste canister, the shells or shells including an exterior surface, aninterior surface, a thickness defined therebetween, and an open weldjoint; forming a full thickness weld in the weld joint; and compressingthe full thickness weld from the interior surface to the exteriorsurface of the shell or shells.

In another aspect, a nuclear waste canister comprises: a weldedcylindrical stainless steel shell having a thickness and including anexterior surface, an interior surface, and an interior space configuredfor storing nuclear waste; the shell comprising at least one fullthickness butt weld extending from the interior surface to the exteriorsurface of the shell, the butt weld defining an adjoining heat affectedzone resulting from formation of the weld; the weld and heat affect zoneof the shell being compacted for an entirety of the thickness of theshell such that a compressive stress field exists in the weld and heataffect zone from the interior surface to the exterior surface.

Embodiments according to additional embodiments of the presentdisclosure provide a method or process for forming a cylindricalshell-type weldment structure in a manner which inhibits the onset ofstress corrosion cracking (SCC). In one implementation, the method maycomprise steps including forming the shell such as via mechanicalrolling, welding open butt joints or seams of the rolled shell, a secondhard (cold) rolling of at least the surfaces of the weld seam or jointregions (i.e. weld and adjacent heat affect zones—HAZ) under relativelyhigh compressive forces produced by the roller, and peening the seam orjoint regions in multiple passes with each pass creating successivelyand progressively wider peening strips or regions along the weld lines.The rolling operations may be performed by a commercial mechanicalroller machine. In the implementation of the method, the second hardrolling may involve re-rolling the entire shell including the welds andHAZ.

In one embodiment, the structure comprises a cylindrical shell formedfrom multiple rolled and welded stainless steel (e.g. austeniticstainless) shell segments which are welded together along acircumferential butt seam to create a welded assembly (i.e. weldment).The multiple shell segments may be necessary depending on the length ofthe structure or vessel being created (shorter structures requiring onlya single shell segment in some instances). Each shell segment comprisesa respective welded longitudinal seam as further described herein.

In one aspect, a method for fabricating a shell weldment includes:providing a cylindrical shell of stainless steel having an open buttseam; welding the butt seam to close the butt seam with a weld, thewelding creating a heat affected zone in the shell adjoining the weld;rolling a weld zone collectively comprising the weld and heat affectedzone under a compressive force after welding; and peening the weld zone.In one embodiment, the shell and weld zone is peened in multiple passes,each of the peening passes being selected to successively andprogressively produce wider peened strips or regions on the shell alongthe weld zone and adjacent portions of the shell.

In another aspect, a method for fabricating a stainless steel shellweldment includes: rolling a flat workpiece of stainless steel to form acylindrical shell, opposing side edges of the shell meeting at an openlongitudinal butt joint; forming a double-V weld in the longitudinalbutt joint to close the joint; rolling the weld and a heat affected zonein the shell adjoining the weld by applying a compressive force with amechanical roller, the weld and heat affected zone collectively defininga weld zone; and peening the weld zone at an exterior surface of theshell.

In another aspect, a method for fabricating a stainless steelcylindrical shell weldment includes: providing a first cylindrical shellsegment and a second cylindrical shell segment, each shell segment beingformed of austenitic stainless steel and comprising having an openlongitudinal butt seam; closing the butt seams of each of the first andsecond shell segments by forming a double-V longitudinal weld in thelongitudinal butt seams, the formation of the longitudinal weldscreating a respective heat affected zone in the shell segments adjoiningeach longitudinal weld; placing the first and second shell segments inabutting end-to-end relationship forming a circumferential butt seamtherebetween; closing the circumferential butt seam by forming adouble-V circumferential weld in the circumferential butt seam, theformation of the circumferential weld creating a respective heataffected zone in the shell segment adjoining the circumferential weld;applying an inward directed compressive force against the longitudinaland circumferential welds and their heat affected zones by a mechanicalroller; and peening the shell segments along the longitudinal andcircumferential welds and their respective heat affected zones inmultiple peening passes, each of the peening passes being selected tosuccessively and progressively produce wider peened strips on the shellin vicinity of the welds and their respective heat affected zones;wherein a residual stress field proximate to the longitudinal andcircumferential welds on an exterior of the shell segments iscompressive after the peening step.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein likeelements are labeled similarly and in which:

FIG. 1 is a partial cross-sectional perspective view of a cask andcanister system for the storage and transport of spent nuclear fuelaccording to the present disclosure;

FIG. 2 is a top perspective view of the canister and bolted lid thereof;

FIG. 3 is a bottom perspective view thereof;

FIG. 4 is a first detail view from FIG. 3;

FIG. 5 is a second detail view from FIG. 3;

FIG. 6 is an exploded perspective view of the canister;

FIG. 7 is a detail view from FIG. 6;

FIG. 8 is a side view of the canister;

FIG. 9 is a top plan view of the canister;

FIG. 10 is a side cross-sectional view of the canister;

FIG. 11 is a detail view taken from FIG. 10;

FIG. 12 is a side cross-sectional view of the lid of the canister;

FIG. 13 is a transverse cross sectional view taken from FIG. 8;

FIG. 14 is a detail view taken from FIG. 13;

FIG. 15 is a top perspective view of a second embodiment of a canisterand bolted lid;

FIG. 16 is a bottom perspective view thereof;

FIG. 17 is an exploded perspective view of the second canister;

FIG. 18 is a detail view from FIG. 17;

FIG. 19 is a side view of the second canister;

FIG. 20 is a top plan view of the second canister;

FIG. 21 is a transverse cross-sectional view taken from FIG. 19;

FIG. 22 is a detail view taken from FIG. 21;

FIG. 23 is a side cross-sectional view of the second canister;

FIG. 24 is a detail view taken from FIG. 23; and

FIG. 25 is a side cross sectional view of the lid of the secondcanister.

FIG. 26A is a cross sectional side view of a known nuclear wastematerial storage canister;

FIG. 26B is a view thereof showing an additional top partial closurering;

FIG. 27A is general schematic diagram of an existing horizontal drystorage system;

FIG. 27B is a general schematic diagram of a existing vertical drystorage system;

FIG. 28 is perspective view of a dry storage system according to thepresent disclosure comprising an inner canister for holding nuclearwaste materials and an outer storage overpack or cask;

FIG. 29 is a partial side cross sectional view of welded joints in FIG.28 between the shell of the canister and the base plate and top lid;

FIG. 30 is a side view thereof showing the addition of a partialsecondary containment barrier in the form of an annular shell skirt;

FIG. 31 is a perspective breakaway view of a double-walled canistersystem comprising an inner canister and outer shell assembly forming afull coverage secondary containment barrier encapsulating the innercanister;

FIG. 32 is a partial side cross sectional view of welded joints used ina canister system having a double-walled shell and single top lid;

FIG. 33 is a partial side cross sectional view of FIG. 32 showing theaddition of a secondary base plate, optional partial closure ring ontop, and optional stay fastener supporting the second base plate;

FIG. 34 is a partial side cross sectional view of welded joints used inthe canister system of FIG. 31 having a double-walled shell andsecondary top lid; and

FIG. 35 is an end view of a horizontal canister having a partial shellskirt on a lower portion of the canister shell.

FIG. 36 is a perspective view of the shell weldment of a nuclear wastecanister for storing spent nuclear fuel;

FIG. 37 is a cross-sectional view taken through line FIG. 37-FIG. 37 inFIG. 36 of a first type of full thickness butt weld used to form theshell;

FIG. 38 is a cross-sectional view taken through line FIG. 37-FIG. 37 inFIG. 36 of a second type of full thickness butt weld used to form theshell;

FIG. 39 is a cross-sectional view taken through line FIG. 37-FIG. 37 inFIG. 36 of a third type of full thickness butt weld used to form theshell;

FIG. 40 is a cross-sectional view taken through line FIG. 37-FIG. 37 inFIG. 36 of a fourth type of full thickness butt weld used to form theshell;

FIG. 41 is a cross-sectional view taken through line FIG. 37-FIG. 37 inFIG. 36 of a fifth type of full thickness butt weld used to form theshell;

FIG. 42 is a cross-sectional view of the butt weld of FIG. 41 showing amechanical weld compaction device used to compress the weld for a fullthickness of the shell and flatten the crown of the weld; and

FIG. 43 is a flow chart showing steps in the process or method forfabricating the nuclear waste canister of FIG. 36.

FIG. 44 is a perspective view of a rolled and welded cylindricalshell-type assembly or weldment in the form of a canister used forstoring spent nuclear fuel and illustrating circumferential andlongitudinal weld joints or seams;

FIG. 45 is a close-up detail of a weld line taken from FIG. 44;

FIG. 46 is a graph showing typical residual stress distribution in thecircumferential direction in the shell of the weldment subsequent to themechanical rolling operation and prior to welding;

FIG. 47 is a transverse cross section through the full penetration depthweld of the weldment taken from FIG. 45;

FIG. 48 is a perspective view showing a peening process used in theformation of the shell weldment of FIG. 44 which employs peening passeson the welds and respective heat affected zones (HAZs); and

FIG. 49 is a flow chart showing an exemplary process or method forforming a cylindrical shell-type weldment according to the presentdisclosure.

All drawings are schematic and not necessarily to scale. Parts shownand/or given a reference numerical designation in one figure may beconsidered to be the same parts where they appear in other figureswithout a numerical designation for brevity unless specifically labeledwith a different part number and described herein. References herein toa whole figure number (e.g. FIG. 1) shall be construed to be a referenceto all subpart figures in the group (e.g. FIGS. 1A, 1B, etc.) unlessotherwise indicated.

DETAILED DESCRIPTION

The features and benefits of the invention are illustrated and describedherein by reference to exemplary embodiments. This description ofexemplary embodiments is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. Accordingly, the disclosure expressly should not belimited to such exemplary embodiments illustrating some possiblenon-limiting combination of features that may exist alone or in othercombinations of features.

In the description of embodiments disclosed herein, any reference todirection or orientation is merely intended for convenience ofdescription and is not intended in any way to limit the scope of thepresent invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivatives thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise.

As used throughout, any ranges disclosed herein are used as shorthandfor describing each and every value that is within the range. Any valuewithin the range can be selected as the terminus of the range. Inaddition, all references cited herein are hereby incorporated byreference in their entireties. In the event of a conflict in adefinition in the present disclosure and that of a cited reference, thepresent disclosure controls.

Multiple inventive concepts are described herein and are distinguishedfrom one another using headers in the description that follows.Specifically, FIGS. 1-25 are relevant to a first inventive concept,FIGS. 26A-35 are relevant to a second inventive concept, FIGS. 36-43 arerelevant to a third inventive concept, and FIGS. 44-49 are relevant to afourth inventive concept. The first through fourth inventive conceptsshould be considered in isolation from one another. It is possible thatthere may be conflicting language or terms used in the description ofthe first through third inventive concepts. For example, it is possiblethat in the description of the first inventive concept a particular termmay be used to have one meaning or definition and that in thedescription of the second inventive concept the same term may be used tohave a different meaning or definition. In the event of such conflictinglanguage, reference should be made to the disclosure of the relevantinventive concept being discussed. Similarly, the section of thedescription describing a particular inventive concept being claimedshould be used to interpret claim language when necessary.

I. Inventive Concept 1

With reference to FIGS. 1-25, a first inventive concept will bedescribed.

FIG. 1 depicts a system for storing and transporting radioactive spentnuclear fuel (SNF) which incorporates a spent fuel canister 100 withcompact bolted lid according to the present disclosure. The systemgenerally includes an outer vertically ventilated overpack (VVO) or cask20 defining a vertical longitudinal axis LA. Cask 20 may have acomposite construction including an outer cylindrical shell 22, innercylindrical shell 23, and radiation shielding material 24 disposed inthe annulus between the shells. In some embodiments, the shieldingmaterial 24 may comprise concrete, lead, boron-containing materials, ora combination of these or other materials effective to block and/orattenuation gamma and neutron radiation emitted by the SNF enclosed bythe cask.

Cask 20 has an elongated body including an open top 27 for insertingcanister 100 into cavity 28, a bottom end 25, cylindrical sidewall 29extending between the ends, and an internal canister cavity 28 definedby the inner shell 23. Cavity 28 extends completely through the caskalong the longitudinal axis LA from the top to bottom end. The cavity 28has dimensions and a transverse cross-sectional area which holds only asingle SNF canister 100 in one embodiment. Cask 20 includes an interiorsurface 23-1 adjacent to canister cavity 28 and opposing exteriorsurface 22-1. Cask 201 may be comprised of a single long cylinder body,or alternatively may be formed by a plurality of axially aligned andvertically stacked cylinder segments seal welded together at the jointsbetween the segments to collectively form the cask body.

The bottom end 25 of cask 20 may be enclosed by circular base 26attached thereto, such as via circumferential seal welding. A canistersupport pad 26-1 of cylindrical shape may be disposed on top of the base26 inside canister cavity 28 to support the spent fuel canister 100. Thepad may be formed of concrete in one embodiment. The cavity 28 of cask20 may be ventilated by ambient cooling air to remove decay heat emittedby the SNF stored inside the canister 100. Cask 20 may therefore includeone or more air inlets 30 communicating with a lower portion of cavity28 and one or more air outlets 31 communicating with an upper portion ofthe cavity. Air flows radially inwards through inlets 30, upwardsthrough the cavity, and radially outwards through outlets 31 (seedirectional airflow arrows). The open top end 27 of the cask 20 isclosed by a removable lid detachably mounted to the cask. The outletducts 31 may be formed between the lid and top of the cask in someembodiments as shown.

FIGS. 1-14 depict spent fuel canister 100 with compact bolted lidaccording to a first embodiment of the present disclosure in furtherdetail. The present canister advantageously comprises a bolted jointbetween the removable top closure lid and the canister body aspreviously described herein, thereby advantageously providing readyaccess to the SNF therein for repackaging or other purposes. The boltedlid joint is further described in the discussion which follows.

Canister 100 includes an elongated cylindrical body 103 comprising asingle shell 106 including an open top 101, an open bottom 102, andsidewall 109 extending therebetween along a vertical longitudinal axisLA of the canister. Axis LA coincides with the geometric verticalcenterline of the canister. Canister 100 further includes a bottombaseplate 110 and a top closure lid 120. Shell 106 may be of monolithicunitary structure in one embodiment formed of a single material.

Shell 106 further includes an inner surface 107 and opposing outersurface 108. A longitudinally-extending fuel cavity 105 extends betweenthe top and bottom ends 101, 102 of the shell along longitudinal axisLA. Cavity 105 is configured to hold a conventional fuel basket 60comprising a prismatic array of longitudinally-extending fuel storagecells 62. Cells 62 of the fuel basket may be defined by a cluster ofelongated tubes 61 (shown), or alternatively interlocked cell dividers.Both designs are used and well known in the art without furtherelaboration necessary. The invention is not limited by the constructionor configuration of the fuel basket used. The cells 62 are eachconfigured for holding a single spent fuel assembly containing pluralused or spent fuel rods removed from the reactor core. Such fuelassemblies are well known in the art without further elaboration. Thespent fuel still emits considerable amounts of decay heat which isremoved by the air-cooled ventilation system of the outer cask 20, aspreviously described herein.

The baseplate 110 is hermetically seal welded to the bottom end 102 ofthe shell 106. In one embodiment, the baseplate may have a largerdiameter than bottom end of the shell such that the baseplate protrudesradially outwards beyond the shell (see, e.g. FIG. 10). This arrangementprotects the longitudinal cooling fins 140 if provided, as furtherdescribed herein. In other embodiments without fins, the baseplate 110may have the same diameter as the bottom end of shell 106 such that theoutward side surface of the baseplate is substantially flush with theouter surface 108 of the shell (see, e.g. FIG. 19).

The first embodiment of a top closure lid 120 variously seen in FIGS.1-14 will now be described in greater detail. FIGS. 10-12 show the lidin larger detail.

Lid 120 may have a multi-stepped construction in one embodimentcomprising a circular body including a top surface 121, bottom surface122, an upper portion 123 adjacent the top surface, lower portion 124adjacent the bottom surface, and an intermediate portion 125. Lowerportion is configured for insertion into the upper portion of cavity 105of canister shell 106 as shown. Accordingly, lower portion has anoutside diameter D4 which is smaller than the inside diameter D3 of atleast the top end 101 of shell 106 measured inside cavity 105.

Intermediate portion 125 protrudes radially outwards beyond the upperand lower portions 123, 124 and defines an upwardly and downwardlyexposed portion thereby forming an annular mounting flange 125-1 whichis part of the bolted lid-to-shell joint. The mounting flange has anoutside diameter D5 which is larger than outside diameter D4 of lowerportion 124 and inside diameter D3 of shell 106. Preferably, in oneembodiment, diameter D5 is substantially the same as outside diameter D1of the shell 106 measured proximate to the top end 101 of shell 106 suchthat flange 125-1 does not protrude substantially beyond the shell inthe radial direction. This advantageously maintains the narrow profileand dimensions of the canister 100 which keeps the inside diameter ofthe outer overpack or cask 20 as smaller as possible. The canister thushas an overall and collective diameter (i.e. D5 and D1) commensuratewith existing SNF canisters having seal welded lids. The underside (i.e.downward facing surface) of mounting flange 125-1 defines an annularsealing surface 125-2 configured to abut and seat on the top end of theshell when the lid is emplaced thereon (see, e.g. FIG. 11). Theinterface between the sealing surface 125-2 and top end 101 of shell 106is preferably one of flat-to-flat.

Lid 120 further includes an annular step-shaped upper shoulder 177 at atransition between the intermediate mounting flange 125-1 and upperportion 123, and an annular step-shaped lower shoulder 128 at atransition between mounting flange and the lower portion 124. Lowershoulder 128 engages the inside edge of the top end of the shell 106inside cavity 105 at to center the lid on the shell. Lower shoulder 128further provides a sealing interface, as further described herein.

Mounting flange 125-1 comprises a plurality of longitudinal bolt throughbores or holes 126 which extend completely through the flange. Boltthrough holes 126 are configured for receiving the at least partiallythreaded shanks 127-1 of threaded fasteners which may be bolts 127 inone embodiment (see, e.g. FIGS. 10-12). Bolts 127 further have adiametrically enlarged tooling head 127-2 configured for engaging andapplying a tool thereto to tighten or loosen the bolts. The underside oftooling heads 127-2 engage the upward facing surface of the mountingflange 125-1 (best shown in FIG. 11). Through holes 126 may beunthreaded in one preferred embodiment, but can be threaded in otherembodiments. Top portion 123 may have any suitable outside diameter D6which is smaller than diameter D5 of the intermediate portion125/mounting flange 125-1 to provide access to the through holes 126 forinserting the bolts therethrough. The lid bolts preferably may beslender, for example about ½-inch diameter in some embodiments with longthreaded length (e.g. at least 4 inches long). By using a greater numberof smaller diameter slender bolts rather than few larger diameter bolts,the radial projection of the lid 1q20 may advantageously be kept to aminimum without adversely affecting the lid-to-shell hermetic seal andin turn minimizes the outside diameter of the canister 100.

Bolt through holes 126 are arranged perimetrically around the mountingflange 125-1 and spaced circumferentially apart covering a full 360degrees of the flange. Preferably, through holes 126 are uniformlyspaced apart to provide even sealing pressure around the entireperimeter of the closure lid 120 when the bolts are tightened. Thecenterline of through holes 126 each defines a bolt axis BA. Theplurality of through holes 126 collectively fall on and define a boltcircle BC intersecting bolt axes BA and extending circumferentiallyaround the mounting flange 125-1.

The top end 101 of shell 106 comprises a plurality of perimetricallyarranged and circumferentially spaced apart threaded sockets or bores130 formed in the top end of the body of the shell 106. Bores 130 arevertically oriented and upwardly open for threadably receiving andengaging the threads on shanks 127-1 of bolts 127. Preferably, at leastthe lower portion of bolt shanks 127-1 are therefore threaded. Bores 130are blind bores meaning the bottom ends of the bores are closed (see,e.g. FIG. 11). Bores 130 fall on the bolt circle BC and thus may each becoaxially aligned with a bolt axis BA of lid through holes 126 by properrotational positioning of the lid on the shell. The bores 130 are formedbetween the inner surface 107 and upper outer surface 108 a of shell 106in the annular mounting boss 132 of the shell which defines topfastening portion 131, as further described below.

To structurally reinforce the canister shell 106 for the bolting, thetop end 101 of shell 106 is radially thickened to form an outwardlyprotruding annular mounting boss 132 integrally formed with the shell.Boss 132 extends around the entire circumference of the upper portion ofthe shell and vertically downwards from top end 101 of the shell 106.Boss 132 may be about 6 inches high in one non-limiting embodiment. Theboss defines a top fastening portion 131 of the shell having a greatertransverse wall thickness T1 (measured perpendicularly to longitudinalaxis LA) than the wall thickness T2 of the portions of the shell belowbetween the bottom end 102 of the shell and the fastening portion 131.This additional thickness provides extra purchase and structurallyreinforces the top end of shell 106 for forming the threaded bores 130.In the illustrated embodiment, the annular mounting boss 132 mayprotrude radially outwards beyond the lower outer surface 108 b of thelower portion of the shell 106 giving the shell a stepped outer surface108. The lower outer surface 108 b is thus recessed radially inwardsfrom the upper outer surface 108 a defined by the boss 132 such thatouter surface 108 a lies in a circular vertical plane which is offsetand spaced farther away from the longitudinal axis LA of shell 106 thanthe lower outer surface 108 b which lies in a different circularvertical plane (see, e.g. FIG. 11).

It bears noting that the mounting boss 132/fastening portion 131 of thecanister shell 106 is distinct from merely forming a conventionalradially projecting flange on the top end of a shell used in bolted headflanged joints in which the shank of the fastener projects completelythrough mating flanges and a nut is threaded onto the bottom exposedshank portion. By contrast, the present mounting boss 132/fasteningportion 131 of shell 106 is a substantially taller/higher thickenedportion at the top end of the shell as shown in FIG. 11 which providesthe important function of structurally reinforcing the shell for formingthe threaded blind bores 130, not merely for accommodating a boltedlid-to-shell joint. Accordingly, embodiments of the present mountingboss 132/fastening portion 131 preferably have a height measuredparallel to longitudinal axis LA which is greater than at least threetimes its radial/transverse wall thickness T1, and some embodimentsgreater than at least five times.

The radially offset between the upper outer surface 108 a and lowerouter surface 108 b of the canister shell 106 defines an outwardly openannular recess 141 extending a full 360 degrees around the circumferenceof the shell in preferred embodiments. The annular recess extends fromthe bottom of the mounting boss 132 to the bottom baseplate 110.

According to another aspect of the invention, the canister 100 maycomprise a plurality of longitudinally-extending cooling fins 140protruding radially outwards from the shell. This provides additionalcooling surface area for dissipating the heat emitted by the SNF storedin side canister 100. The fins are arranged perimetrically around theentire circumference of the shell 106 and spaced circumferentiallyapart, preferably at regular intervals with uniform spacingtherebetween. The fins have a vertical length which extends for amajority of the vertical length of the shell to maximize the effectiveheat transfer area of the canister. Fins 140 may be formed integrallywith the shell as a monolithic unitary structural portion thereof usinga thick plate stock for the shell machined to form the fins. A typicalplate stock may be 1¼-inch thick with machined rectangular fins ¾-inchhigh by ½-inch thick space at a 1¼-inch pitch around the circumferenceof the canister shell 106. Alternatively, the fins 140 may be discretestructures welded to the outer surface 108 of the shell 106. Fins 140may be longitudinally straight structures including opposing side majorsurfaces and a straight vertical longitudinal edge as shown. In oneembodiment, the fins 140 may have a wedge-shaped transverse crosssection in which the side major surfaces converge moving radiallyoutwards (best shown in FIG. 14). In other possible, embodiments, theside major surfaces may be parallel to each other. In one preferablyarrangement, the fins 140 may be disposed on the lower outer surface 108b of shell 106 below the enlarged mounting boss 132-fastening portion131 of the shell. Fins 140 extend vertically from the bottom of mountingboss 132 to the bottom baseplate 110 of the canister.

In one preferred but non-limiting arrangement, the cooling fins 140 maybe completely disposed within the outwardly open annular recess 141 ofthe shell 106. This protects the fins from damage during handling andtransport of the canister and advantageously maintain the desired smalloutside diameter of the canister 100 for storage in the outer radiationshielded cask 20. Accordingly, in this embodiment, fins 140 do notprotrude radially outwards beyond the upper reinforced fastening portion131 (i.e. boss 132) of the shell 106. The fins further may additionallynot protrude radially beyond the mounting flange 125 of lid 120. And insome embodiments, the fins may further also not protrude radially beyondthe baseplate 110 of the canister 100 to maximize protection of the finsfrom structural damage during handling of the canister and minimize theradial projection of the fins to maintain the small canister diameter.

In one embodiment, the top ends of the fins 140 may abut the underside(i.e. downward facing surface) of the annular boss 132 (see, e.g. FIG.11), or alternatively terminate proximate thereto without contact. Theopposite bottom ends of the fins 140 may terminate at a point proximateto but slightly spaced above the baseplate 110 to provide access forcircumferentially seal welding the baseplate to bottom end 102 of theshell (see, e.g. FIGS. 5 and 10).

For canisters containing a moderate heat load, its finned surface may besufficiently effective to keep the peak fuel cladding temperature of theSNF inside the canister moderate (defined as <300 degrees C.) and thusadvantageously permit the use of a less expensive inert gas such asnitrogen in lieu of helium, as the fill gas in the canister.

Any suitable metallic materials may be used for constructing the lid120, shell 106, plate 108, and fins 140. In one embodiment, stainlesssteel may be used for corrosion protection. Welding-friendlycopper-nickel alloys and duplex stainless steel are also acceptablematerials.

The longitudinal fin 140 arrangement discussed above applies tovertically stored canisters such as in the HI-STORM storage systemavailable from Holtec International. In storage systems that employhorizontally oriented canisters, the direction of the fin on the shellmust be circumferential (preferably, helical) to effect improvement inheat rejection. Circumferentially oriented fins can also be effectivelyutilized to eliminate hide-out crevices formed at the junction of thehorizontal canister and rails that support it.

FIGS. 10 and 11 show the lid 120 fully seated, bolted, and sealed to thetop fastening portion 131 of canister shell 106. The outer surface 125-3of the mounting flange 125 of lid 120 does not project radially outwardsbeyond the upper outer surface 108 a formed by the top fastening portion131 defined by the annular mounting boss 132 of the shell. Accordingly,surfaces 125-3 and 108 a lies in the same circular vertical plane Vp.The longitudinal edges 142 of cooling fins 140 occupying the annularrecess 141 on the shell 106 do not protrude radially outwards beyond thetop fastening portion 131 or lid 120; the edges also lying in the samevertical plane Vp. Each mounting bolt 127 passes vertically through itsrespective bolt through hole 126 in the intermediate mounting flange 125of the lid and directly threadably engages the shell via the threadedbores 130 formed through the upward facing annular end surface 111 atthe top end 101 of the shell.

In order to keep the outer diameter of the canister assembly to minimumfor providing the desired compact small profile lid construction whichemulates existing small profile welded rather than bolted canister lidsfor packaging in radiation shielded outer overpacks such as cask 20previously described herein, special spatial relationships are createdby the present lid as shown in FIG. 11. The radial distance R1 betweenthe longitudinal axis LA of canister 100 and bolt axes BA/bolt circle BCis less than both the radial distance R6 between upper outer surface 108a of shell 106 and axis LA, and radial distance R3 between outer surface125-3 of lid mounting flange 125 and axis LA. Radial distance R1 howeveris greater than radial distance R5 between axis LA and inner surface 107of shell 106, and radial distance R4 between axis LA and outer surface124-1 of lid lower portion 124 inside shell cavity 105. Radial distanceR1 is also greater than radial distance R7 between axis LA and outerlower surface 108 b of shell 106. Radial distance R2 betweenlongitudinal axis LA and outer surface 123-1 of lid upper portion 123 isless than R1, R3, and R6, but greater than R4 and R5 in one embodiment.R2 may be substantially the same as R7 in one embodiment.

By keeping the outer diameter of the canister as small as possible, theouter transport/storage cask 20 dimensions are advantageously minimizedwhich reduces fabrication costs and facilitates handling the large heavycasks with lifting equipment.

To seal the lid 120 to shell 106, a pair of circumferential seals isprovided including an annular inner seal 150 and annular outer seal 151.Inner seal 150 seals the lower portion 124 of the lid to the innersurface 107 of shell 106. A piston type seal arrangement may be providedas shown comprising an outward facing annular piston groove 152 formedin the outer surface 124-1 of lid lower portion 124 in which inner seal150 is retained. When the lid 120 is placed on the top fastening portion131 of the shell, the smaller diameter lid lower portion 124 is insertedinto inside the upper portion of shell cavity 105. Inner seal 150 slidesdown along the inner surface 107 of the shell until the lid is fullyseated on the canister.

The circumferential outer seal 151 seals the step-shaped lower shoulder128 of lid 120 to the top annular end surface 108 of the shell 106. Anannular groove 153 is formed at the innermost corner edge of end surface108 which retains the outer seal 151. The inner and outer seals 150, 151provide two independent high integrity leak barriers advantageouslycreating redundant protection against leakage of gaseous matter frominside the canister 100. Any suitable annular seals may be used. In oneembodiment, the seals may be O-rings formed of a suitable sealingmaterial such as without limitation flexible elastomeric materials.

FIGS. 15-25 depict a spent nuclear fuel (SNF) canister 200 with compactbolted lid according to a second embodiment of the present disclosure infurther detail. SNF canister 200 is similar to canister 100. Similarparts will not be described in detail or numbered in the figures for thesake of brevity. There are some notable differences in design. Forexample, the shell 206 of canister 200 is substantially similar to shell106 of canister 100 with exception that is does not have a step-shapedouter surface with annular recess. Instead, the inner surface of theshell is step shaped as further described below. In addition, canister200 may be finless as shown, or alternatively may be equipped withexternal cooling fins if heat emitted by the SNF is considerable. Topclosure lid 220 has a different configuration than lid 120 of canister100; however, it retains the small profile bolted joint to the canistershell as further described below. In addition, lid 220 of canister 200has a different sealing arrangement.

Referring now to FIGS. 15-25, canister 200 includes an elongatedcylindrical body 203 comprising a single shell 206 including an open top201, an open bottom 202, and sidewall 209 extending therebetween along avertical longitudinal axis LA of the canister. Axis LA coincides withthe geometric vertical centerline of the canister. Canister 200 furtherincludes a bottom baseplate 210 and a top closure lid 220. In thisfinless embodiment of a shell 206, the baseplate preferably does notprotrudes radially outwards beyond the lower portion of the shell tokeep the outside diameter of the canister to a minimum for placementinside the outer radiation shielded overpack or cask 20. Shell 206 maybe of monolithic unitary structure in one embodiment formed of a singlematerial.

Shell 206 further includes an inner surface 207 and opposing outersurface 208. A longitudinally-extending fuel cavity 205 extends betweenthe top and bottom ends 201, 202 of the shell along longitudinal axisLA. Cavity 205 is similarly configured to that of canister 100 to hold aconventional fuel basket 60 comprising a prismatic array oflongitudinally-extending fuel storage cells 62, as previously describedherein.

To structurally reinforce the canister shell 206 for the bolting, thetop end 201 of shell 206 is radially thickened but in an inwardsdirection creates a uniform outer surface 208 but a step-shaped innersurface 207. This is dissimilar to shell 106 of canister 100 previouslydescribed herein which is radially thickened in an outward direction.Shell 206 therefore comprises an inwardly protruding annular mountingboss 232 integrally formed with the shell 206 at its top end 201. Boss206 extends around the entire circumference of the upper portion of theshell. The boss defines top fastening portion 231 of the shell 206having a greater transverse wall thickness T3 than the wall thickness T4of the portions of the shell below between the bottom end 202 of theshell and the fastening portion 231. A plurality of upwardly openthreaded bores 230 similar to bores 130 previously described herein arearranged and spaced circumferentially around the top end 201 of shell206. Bores 230 penetrate upward facing annular end surface 211 of theshell.

Referring particularly to FIGS. 23-25, the present lid 220 may have astepped construction in one embodiment comprising a circular bodyincluding a top surface 221, bottom surface 222, an upper portion 223adjacent the top surface, and a lower portion defining a radiallyprotruding annular mounting flange 225 which is part of the boltedlid-to-shell joint. 124 adjacent the bottom surface, and in immediateportion 125. The mounting flange has an outside diameter D10 which islarger than outside diameter D11 of upper portion 223 of lower portion124 and inside diameter D13 at the fastening portion 232 of shell 106.An annular step 270 is formed between the upper portion and mountingflange. Preferably, in one embodiment, diameter D10 is substantially thesame as outside diameter D14 of the shell 206 such that flange 225 doesnot protrude substantially outwards beyond the shell in the radialdirection. This advantageously maintains the narrow profile anddimensions of the canister 200 which keeps the inside diameter of theouter overpack or cask 20 as smaller as possible. The canister thus hasan overall and collective diameter (i.e. D11 and D14) commensurate withexisting SNF canisters having seal welded lids. The underside (i.e.downward facing surface) of mounting flange 225 defines an annularsealing surface 225-2 configured for positioning on the top end surface211 of the shell when the lid is emplaced thereon (see, e.g. FIG. 24).The interface between the sealing surface 225-2 and end surface 211 ispreferably one of flat-to-flat for accommodating annular outer seal 251.Seal 251 may be a planar self-energizing or raised face gasket in oneembodiment that forms the outermost secondary confinement barrier toprevent gaseous products from leaking from the canister cavity 205 tothe outer environment. Any suitable metallic or non-metallic sealmaterial may be used.

In the present lid 220 design, it bears noting that no portion of thelid protrudes downwards into the top portion of the canister cavity 205in contrast to lid 120 previously described herein. Instead, a circulardisk-shaped shield plate 260 is provided which sits immediately down andinside the top end of the cavity 205 as shown in FIGS. 23-24. Thecircumferential peripheral edge of the shield plate 260 is supported byan upward facing annular support surface 261 defined by an annularstep-shaped shoulder formed in the upper inner surface 207 a of shell206 proximate to its top end 201, but spaced vertically downwardtherefrom as shown. The support surface 261 is thus formed in theradially thickened upper fastening portion 232 of the shell. Shieldplate 260 forms part of the primary containment boundary of the canister200. The shield plate may be sealed by an inner seal which may comprisea circular disk-shaped diaphragm seal 250 disposed between the shieldand bottom surface 222 of the lid 200. Both the shield plate anddiaphragm seal may be formed of a suitable metallic material, such asstainless steel in one embodiment.

Canister 200 further includes Lid 120 further includes an annularstep-shaped upper shoulder 127 at a transition between the intermediatemounting flange 125-1 and upper portion 123, and an annular step-shapedlower shoulder 128 at a transition between mounting flange and the lowerportion 124. Lower shoulder 128 engages the inside edge of the top endof the shell 106 inside cavity 105 at to center the lid on the shell.Lower shoulder 128 further provides a sealing interface, as furtherdescribed herein.

Mounting flange 125-1 comprises a plurality of longitudinal bolt throughbores or holes 126 which extend completely through the flange. Boltthrough holes 126 are configured for receiving the at least partiallythreaded shanks 127-1 of threaded fasteners which may be bolts 127 inone embodiment (see, e.g. FIGS. 10-12). Bolts 127 further have adiametrically enlarged tooling head 127-2 configured for engaging andapplying a tool thereto to tighten or loosen the bolts. The underside oftooling heads 127-2 engage the upward facing surface of the mountingflange 125-1 (best shown in FIG. 11). Through holes 126 may beunthreaded in one preferred embodiment, but can be threaded in otherembodiments. Top portion 123 may have any suitable outside diameter D6which is smaller than diameter D5 of the intermediate portion125/mounting flange 125-1 to provide access to the through holes 126 forinserting the bolts therethrough.

FIGS. 23 and 24 shows the lid 220 fully seated, bolted, and sealed tothe top fastening portion 232 of canister shell 106. The outer surface225-1 of the mounting flange 225 of lid 220 does not project radiallyoutwards beyond the outer surface 108 formed by the top fasteningportion 231 defined by the annular mounting boss 232 of the shell.Accordingly, surfaces 125-1 and 208 lie in the same circular verticalplane Vp. Each mounting bolt 127 passes vertically through itsrespective bolt through hole 226 in the mounting flange 225 of the lidand directly threadably engages the shell via the threaded bores 230formed through the upward facing annular end surface 211 at the top end201 of the shell. Shield plate 260 is recessed in the top end 201 ofshell 206 inside cavity 205 such that the top surface of the shieldplate does not protrude upwards beyond the top end 201 of the shell. Theinner diaphragm seal 250 lies in the same horizontal sealing plane asthe outer annular seal 251.

Special spatial relationships are created by the present lid 220 asshown in FIG. 24 to maintain the compact lid and canister profiles. Theradial distance R10 between the longitudinal axis LA of canister 200 andbolt axes BA/bolt circle BC is less than both the radial distance R11between outer surface 208 of shell 206 and axis LA, and radial distanceR13 between outer surface 225-1 of lid mounting flange 225 and axis LA.R13 and R11 may be substantially the same providing a flush lid to shelltransition and outer surfaces. Radial distance Radial distance R10 maybe substantially the same are radial distance R12 between axis LA andthe lower inner surface 207 b of shell 206.

II. Inventive Concept 2

With reference to FIGS. 26A-35, a second inventive concept will bedescribed.

FIG. 28 shows a vertical dry storage system according to the presentdisclosure for long term storage of radioactive nuclear waste materialssuch as spent nuclear fuel (SNF) or other waste materials which may havebeen radioactively contaminated such as in a nuclear power generationfacility. The dry storage system comprises an inner canister 1020 whichis removably inserted inside an outer radioactively shielded storageoverpack or cask 1030. The cask and canister are configured for uprightvertical storage of the nuclear waste materials.

The storage cask 1030 is a heavy-walled steel and concrete cylindricalvessel having a composite construction comprising a cylindrical innermetal shell 1035, a cylindrical outer metal shell 1036, and acylindrical intermediate concrete structure 1037 disposed between themetal shells. A removable lid 1031 closes the open top end 1038 of thecask 1030 and a bottom plate 1033 closes the opposing open bottom end1039. The cask is a generally massive and heavy structure which absorbsthe neutron and gamma fluxes emanating from the spent nuclear fuel (SNF)stored in the canister 1020. Cask 1030 defines an internal cavity 1034extending between opposing top and bottom ends 1038, 1039 of the innershell 1035 which is sized and configured to fit the canister 1020therein.

Preferably, an annular gap 1040 is formed between the inner shells 1035and the canister 1020 which provides a pathway for cooling air to flowupwards and remove heat generated from the radioactively decaying spentnuclear fuel inside the canister 1020. One or more circumferentiallyspaced apart cooling air inlets 1041 and cooling air outlets 1042 areformed at the bottom and top ends 1039, 1038 of cask 1030 which are influid communication with the annular gap 1040. In operation, air in theannular gap 1040 heated by the SNF inside canister 1020 flows upwardsvia natural circulation between the canister 1020 and cask 1030. Theheated cooling air is discharged radially and/or upwardly outwards fromthe upper portion of the gap through the air outlets 1042, therebydrawing fresh ambient cool air radially and/or upwardly inwards throughthe air inlets 1041 into the lower portion of the annular gap 1040 (seedirectional cooling air flow arrows showing flow path).

Referring to FIGS. 28-30, canister 1020 has an elongated cylindricalhollow body comprising a tubular shell 1050 defining an internal cavity1051 for storing nuclear waste material. A SNF basket such as disclosedin U.S. Pat. No. 5,898,747 (incorporated herein by reference in itsentirety) may be disposed in the cavity to facilitate storage of theused fuel rods. A top lid 1052 is sealably attached to a top end 1053 ofthe shell 1050 and an end closure 1054 is sealably attached to anopposing bottom end 1057 of the shell. The lid and end closurepreferably may be circular in shape to conform to the circular tubularshell. In some arrangements, the lid 1052 may be partially or completelyreceived into the cavity 1051 through top end 1053 of shell 1050 asshown. The upwards facing top surface of the lid 1052 may therefore beflush with the top end of the shell in one implementation as best shownin FIGS. 29 and 30, or in other configurations may recessed therein orextend partially above the top end of the shell. The canister 1020 has afully welded construction which hermetically seals the internal cavity1051 storing the spent nuclear fuel therein.

In one embodiment, the bottom end closure 1054 of canister 1020comprises a primary base plate 1055 and a peripheral upturned annularclosure flange 1056 disposed on the annular peripheral area or portionof the base plate. The annular edge 1059 of the flange is horizontallyorientated and defines an abutment edge 1059 having an end surface whichfaces upwards for forming a butt joint with the downward facing endsurface of the bottom end of canister shell 1050, as further describedherein. The edge 1059 of flange 1056 may be substantially flush with thetop surface of the base plate 1055 in some embodiments as illustrated inFIGS. 29 and 30 lying the same horizontal surface plane. However, inother possible embodiments the flange 1056 may project upwards for ashort distance beyond the horizontal plane defined by the top surface ofthe base plate 1055. Flange 1056 is preferably formed integrally withthe base plate 1055 as a unitary structural part thereof so that thereis a single annular joint between the shell 1050 and base plate.However, in other possible embodiments, the flange may be a separatecomponent such as a ring welded to the base plate but with the sameoverall configurations as shown herein.

In one embodiment, the lid 1052 and base plate 1055 of canister 1020 maypreferably be substantially thicker than the shell 1050 followingstandard construction approaches used to nuclear waste material drystorage canisters. The shell, lid, and bottom end closure of canister1020 are preferably made of a suitable corrosion resistant metal, suchas without limitation stainless steel including ferritic, austenitic(e.g. 316 L and 317 L), and Duplex (e.g. Alloy S31803 and S32205)stainless steels. Duplex stainless is a dual-phase stainless steel witha grain structure that comprises both ferrite and austenite. Othersuitable metallic materials including boron containing metals may beused for fabricating the SNF storage canisters.

With continuing reference to FIGS. 28-30, a circumferentially-extendingbutt joint 1058 may be formed between the horizontally oriented edge1059 of the peripheral annular closure flange 1056 and the bottom end1057 of tubular shell 1050. A circumferentially-extending butt weld 1060permanently hermetically seals and joins the bottom end closure 1054 tothe shell of canister 1020. Advantageously, the standard corner jointconfiguration and partial thickness groove/edge welds used in priorcanister shell/base plate junctions (see, e.g. FIG. 26A) are thereforereplaced by the full thickness butt weld configuration which extendscompletely through the joint from interior to exterior as shown forexample in FIGS. 29 and 30. The butt weld provides several benefits.First, the butt weld joint 1058 facilitates improved non-destructionexamination (NDE) testing of the shell/base plate joint by suchtechniques as radiography and ultrasonic (volumetric) as may be requiredby government regulatory agencies. The integrity of prior art edge weldsdue to their geometric configuration of such joints are less amenable toNDE techniques used to identify possible subsurface irregularities orinclusions such as radiography and ultrasonic testing. In addition, thebutt weld joint disclosed herein also helps minimize the residualstresses trapped in the weld which can serve as nuclei for stresscorrosion cracking (SCC) in certain ambient environments.

The butt joint 1058 is preferably positioned proximate to base plate1055. In one embodiment, a circumferentially-extending upwardly concavesurface depression 1084 may be formed in the top surface of base plate1055 between the base plate and annular closure flange adjacent to theannular flange 1056 and butt joint 1058 to allow the joint to be locatedas close as possible to the top surface horizontal plane of the baseplate as in the illustrated embodiment of FIGS. 29 and 30. The edge 1059of the annular flange and butt joint are therefore substantially flushin position with the top surface of the base plate 1055. The depression1084 creates greater access to the interior side of the butt joint 1058to facilitate both forming the butt welded joint and subsequent NDE ofweld in such a closely coupled arrangement. In other less preferred butstill satisfactory embodiments, however, the annular closure flange 1056of bottom end closure 1054 may protrude upwards for a short distanceabove the top surface of the base plate 1055 (not shown) forming a sortof short vertical circular stub wall which is then butt welded to thebottom end shell 1050. Either arrangement may be used.

The annular peripheral edge of the top lid 1052 may be fully welded andhermetically sealed to the top end 1053 of the canister shell 1050 usingany suitable type weld such as a groove weld 1061. In one embodimentshown in FIGS. 29 and 30, the weld 1061 which extends circumferentiallyaround the entire top end of the canister may be a bevel groove weld asshown; however, other suitable types of groove or other welds may beused depending on the lid to canister interface configuration. It iswell within the ambit of those skilled in the art to select anappropriate type of weld, weld filler material, and correspondinglid/shell edge profile as needed.

Because the lower portion of the canister 1020 exposed to the colder airis the most susceptible location to stress corrosion cracking (SCC) asdiscussed above, the present disclosure provides several techniques formitigating the initiation or propagation of this failure mechanism. Invertical dry storage systems, the bottom end region of the canister 1020is the problematic area for SCC since the coldest cooling air enters theouter overpack or cask 1030 at this location (see, e.g. FIG. 28).

For vertical dry storage canisters 1020, a double-walled/multi-walledcanister construction may be used to combat SCC. The multi-wallcanister, as the name implies, comprises designing the canister with twodiscrete sealed enclosures that are configured such that a thru-wallcrack originating in one will not propagate into the other, therebyproviding a redundant or back-up sealed shell to guard against possibleradioactive contamination leakage from the canister. For this purpose,the two shells are preferably located intimately close to each otherforming as extremely small interstitial space between them but are notphysically bonded together by explosive or other techniques which wouldpermit a crack originating in one shell to migrate into the next shell.As further described below, double wall shell construction geometry atvarious levels of enveloping the radioactive material confinementboundary are presented in the embodiments which follow.

FIG. 31 depicts a first skirted multi-wall canister constructioncomprising partial double wall shell coverage for a vertical dry storagecanister 1020 usable in the vertical dry storage system shown in FIG.27B above. In this embodiment, a partial annular second outer shellskirt 1070 envelopes only a lower region or portion of the inner shell1050 to target the coldest area of the canister 1020 as a finalcontainment barrier. Skirt 1070 extends around the full circumference ofthe shell 1050. Preferably, the skirt 1070 has a height less than thefull height of the inner shell 1050, and more preferably less than orequal to half the full height of the inner shell. The skirt 1070 has anarcuate concave shape which conforms to the diameter and geometry of theinner canister shell 1050. In one embodiment, a plurality of welds areused to hermetically seal the outer shell skirt 1070 to the lowerportion of the inner shell 1050. This may include a plurality ofcircumferentially spaced apart plug welds 1062 formed around the skirtto secure it preferably to the base plate 1055 of the canister 1020which has a greater material thickness than the inner shell providing amore robust connection. The annular-shaped top end 1074 of the shellskirt 1070 may be sealed to the inner shell 1050 via a fullcircumferential fillet weld 1063. The annular-shaped bottom end 1075 ofthe skirt may be sealed to base plate 1055 via another fullcircumferential fillet weld 1063. Other types of welds may be used inother embodiments. The outer shell skirt 1070 is hermetically sealed tothe inner shell 1050 to form a secondary pressure retention boundarycorresponding to the extent of the inner shell covered by the skirt.

FIG. 35 shows a partial coverage shell skirt 1700 adapted for ahorizontal dry storage canister 1200 such as those used in thehorizontal dry nuclear waste material storage system shown in FIG. 27Aabove. The skirt 1700 has a concave shape which conforms to the diameterand geometry of the storage canister shell. The skirt 1700 has acircumferential extent less than the fully circumference of the innershell 150 and preferably a length at least coextensive with the lengthof the inner shell. In one embodiment, the skirt 1700 may cover theentire lower half of the canister 1200 as shown, or in other embodimentsa greater or less circumferential extend of the canister shell. Filletwelds 1063 or other suitable welds may be used to hermetically seal theouter shell skirt 1700 to the canister 1200.

FIGS. 31-34 depict examples of a more complete coverage multi-wallcanister construction each commonly comprising a complete double wallshell system as a starting point. In the embodiment of FIGS. 33-33, atubular second outer shell 1071 adjoins and envelopes the entire innershell 1050 for at least the full height of the inner shell 1050extending from at least the top end 1053 to bottom end 1057 of the innershell. The outer shell 1071 defines an internal cavity 1076 configuredand dimensioned to hold inner shell 1050. Outer shell 1071 is preferablyin very close proximity to the inner shell 1050 as much as practicableto minimize the size of any annular interstitial space formedtherebetween to minimize the amount of air trapped therein which may beevacuated and filled with an inert gas.

Outer shell 1071 has a greater height than the inner shell 1050 in oneembodiment such that the lower end of the outer shell extends below thelower end of the inner shell for welding to the base plate 1055 of theinner shell. The top ends of the inner and outer shells 1050, 1071 maybe at the same elevation or height as shown. The annular-shaped top endof the outer shell 1071 may be sealed to the inner shell 1050 via a fullcircumferential groove weld 1064 between the top ends 1072, 1053 of theouter and inner shells respectively. The annular-shaped bottom end 1073of the outer shell 1071 may be sealed to base plate 1055 via the sameplug welds 1062 and full circumferential fillet welds 1063 describedabove (see, e.g. FIG. 32). Other types of welds may be used in otherembodiments. The outer shell 1071 is hermetically sealed to the innershell 1050 to form a secondary pressure retention boundary correspondingto the full extent of the inner shell 1050 covered.

Full shell and base plate coverage may be provided in an alternativeembodiment of a multi-wall canister 1020 as shown in FIGS. 32 and 33. Inthis configuration, the primary base plate 1055 welded to primary innershell 1050 may be covered/enclosed and protected by a secondary baseplate 1082 welded to the secondary outer shell 1071. This embodimentincludes a single primary top lid 1052. Optionally, the top lid 1052 mayalso include annular closure ring 1080 as already described above. Theouter secondary base plate 1082 adjoins and completely covers thedownward facing bottom surface of the inner primary base plate 1055being in very close proximity thereto. The interstitial space betweenthe inner and outer boundaries of the shells 1050, 1071 and base plates1055, 1082 may be evacuated and filled with an inert gas (e.g. helium ornitrogen). The outer shell 1071 may be hermetically sealed to thesecondary base plate 1082 by a circumferentially-extending suitable weldsuch as a groove weld 1061. In this embodiment, the bottom end 1073 ofouter shell 1071 may be coextensive with the bottom surface of the innerbase plate 1055 as shown in FIG. 33.

In the embodiment of FIG. 33, an annular top closure ring 1080 mayoptionally be welded to the top lid 1052 and top ends 1072, 1053 of theinner and outer shells 1050, 1071 respectively. Any suitable type ofweld may be provided. In one embodiment, a full circumferential filletweld 1063 may be provided on the inner and outer sides of the closurering 1080 to hermetically seal the ring to the canister 1020. The ring1080 preferably has a sufficient width to cover and closes allthrough-ports 1081 or other apertures that extend completely through thelid into the cavity 1051 of the canister which may be used to evacuateair from and dry the interior of the canister during the canister dryingprocess described above.

In some embodiments, the secondary base plate 1082 may optionally bebuttressed by a plurality of stay fasteners 1083 of any suitable type,as shown in FIG. 33. Fasteners 1083 extend completely through the outerbase plate 1082 and only partially into the inner base plate 1055 to adepth which provides sufficient purchase to secure the fasteners inthereto. In one embodiment, threaded fasteners such as bolts or screwmay be used to reinforce the attachment between the inner and outer baseplates 1055, 1082. The stay fasteners may be necessary if the secondarybase plate 1082 and its junction with the outer shell 1071 are notstructurally sufficient to withstand the internal pressure under thehypothetical scenario in which the inner confinement shell is assumed tohave developed a through leak. This forms a secondary or backup pressureretention barrier.

In an alternative embodiment and variation of the multi-wall (doubleshell) canister construction shown in FIGS. 31 and 34, complete top,bottom, and side confinement boundary coverage may be provided for theinner shell, its base plate, and its bottom closure assembly. Thisconfiguration envisages surrounding the entire primary confinementboundary of canister 1020 formed by inner shell 1050 by a completesecondary confinement boundary. An outer secondary top lid 1079 issealably welded along its peripheral edge to the top end 1077 of theouter shell 1071, which thereby fully covers and encloses the top lid1052 on the inner shell 1050. A full circumferential groove weld 1061may be used to seal the lid 1079 to the shell 1071 (see, e.g. FIG. 34).The top lid 1079 is preferably in close proximity to the top lid 1052.In this construction, the outer shell 1071 extends above the top end1053 of the inner shell 1050 (see, e.g. FIG. 31) in contrast to thearrangement without a secondary top lid 1079 in which the ends of theinner and outer shells are flush with each other (see, e.g. FIGS. 32 and33). The primary base plate 1055 at the bottom end of the inner shell1050 is already covered and enclosed by the secondary base plate 1082 asdescribed herein.

As a further defense-in-depth measure, the interstitial space betweenthe two walls of the inner and outer shells 1050, 1071 in the foregoingmulti-wall constructions may be evacuated and filled with an inert gassuch as without limitation helium or nitrogen. It also bears noting thatwhether a partial or full outer shell is provided for a multi-wallcanister 1020, the cavity 1034 of the outer storage cask 30 is stillsized to insert and accommodate the canister therein in a manner whichmaintains the annular gap 1040 between the canister and cask for naturalconvective cooling.

It will be appreciated that any of the foregoing double-walled orskirted canister assembles may be stored in the overpack or cask 1030shown in FIG. 28 by sizing the cavity 1034 appropriately to accommodatethe larger diameter portions of the double-walled or skirted canisters.It will also be appreciated that all welds disclose herein with theexception of plug welds 1062 are full circumferential welds extendingcompletely around the canister and its shell, lids, or base plates asapplicable to form a fully hermetically sealed pressure boundaries.

III. Inventive Concept 3

With reference to FIGS. 36-43, a third inventive concept will bedescribed.

To devise the remedial measures, it is important to recognize that theweld seams in the canister are locations of the highest tensile stress.The through-thickness welds extending from the interior to exterior ofthe shell plates or sheets made to join the shell courses, as explainedlater, are intrinsically vulnerable locations where the stress field onthe surface is severely tensile. Predictably, these locations are primecandidates for initiation of stress corrosion cracking (SCC). The provenmethod to deal with this problem is surface “peening” which involvesapplying concentrated impulsive pressure on the target surface to createa layer of compressive in-plane stress which serves as an armor againstnucleation of SCC sites under a sustained exposure to salt air. However,surface peening can only produce a compressive layer to a limited depthin the shell base material such as for example as deep as 4 mms which,as test data shows, may be generally adequate to protect against SCC inmany ambient environments. In certain harsh marine environments,however, peening alone may be not provide sufficient protection andservice life.

Although surface peening can be used as a generally effective means tomake the stress field in the welded region compressive over its externalsurface in the weld zone subsequent to the manufacturing of thecanister, the techniques presented in this disclosure can be used assupplemental to peening or as stand-alone measures to further enhanceprotection against SCC and extend the service life of the MPC and otherweldments.

Referring to FIGS. 36 and 37, the method or process for fabricating awelded spent nuclear fuel (SNF) canister 2020 to mitigate or postponethe onset of SCC begins with first forming the circumferential and/orlongitudinal welds 2026 at the seams or joints in the shell 2028 of theSNF canister. The shell 2028 of a nuclear waste canister 2020 isgenerally formed from plates or sheets of stainless steel such asaustenitic stainless steel having a thickness T. As shown, the shell2028 of canister 2020 may typically be a welded structure formed from aplurality of linearly stacked cylindrical shell courses or segmentswelded together at circumferential joints 2023. Each shell segment mayin turn be a welded component including one or more longitudinal joints2021 at which opposing longitudinal ends or edges of the shell segmentare welded to seal the segment. The welds 2026 used for joining sectionsof the shell at either the longitudinal joints 2021 or circumferentialjoints 2023 are full thickness through welds extending from the interiorsurface 2022 of the canister to the exterior surface 2024. The term“exterior surface” means the surface of the weldment that will beexposed to a corrosive halide or chloride environment that has thepotential for initiating SCC. The canister shell 2028 has a cylindricalshape that defines an interior space 2025 configured for holding a fuelstorage basket 2060 having a multiplicity of open cells 2062 that holdthe spent nuclear fuel assemblies, as is well known in the art. Spentfuel canisters having such basket structures are disclosed incommonly-owned U.S. Pat. No. 9,748,009, which is incorporated herein byreference in its entirety. All canister butt welds 2026 are preferablyhermetically sealed welds to prevent escape of radioactive contaminantsto the outer environment from nuclear fuel stored inside the canister2020.

The first measure proposed herein to mitigate the onset of SCC isselecting a type of weld 2026 and/or forming the weld in a manner thatminimizes the heat input to the shell 2028. This will reduce the size ofthe heat affected zone (HAZ) which creates the initiation sites for SCCdue to the tensile stresses created in the HAZ resulting from heatingthe shell base material during the welding process. FIG. 37 shows aconventional double bevel joint or double V-groove butt weld that may beused with aspects of the present invention and process for forming awelded SNF canister. The weld seam or joint shown may be longitudinal orcircumferential joint 2021 or 2023. Two opposing shell ends or edges2030 are shown in spaced apart relationship forming an open weld joint2021 or 2023. The joint thus formed may be linear in one embodiment(see, e.g. FIG. 36) whether longitudinally or circumferentiallyextending. The edges 2030 may be substantially coplanar and parallel toeach other (allowing for shop fabrication dimensional tolerances) toform a generally uniform gap therebetween for receiving the weldmaterial deposited by a consumable welding rod, wire, or electrode (notshown). In FIG. 37, the shell end weld preparations for welding areconfigured to create a profile that forms a standard double V-groove asshown.

According to one aspect of the present invention, if a double beveljoint or double V-groove weld is used as shown in FIG. 37, the bevel atthe interior surface 2022 of the shell 2028 preferably may be madelarger instead and the bevel at the exterior surface 2024 may be made assmall as possible when the joint is welded. FIG. 38 shows such aconfiguration. The enlarged interior weld mass 2032 of the weld isnoticeably larger than the enlarged exterior weld mass 2034.Advantageously, this reduces the heat input during the welding processto make the smaller exterior weld bevel, thereby resulting in smallertensile stress field in the HAZ on the exterior which is the potentialinitiation site for SCC that is exposed to the corrosive salt-ladenmarine environment. As opposed to the completely symmetrical shell weldend preparations of the standard double V-groove weld shown in FIG. 37,the end preparations in FIG. 38 are asymmetrical such that the exteriorV-groove is smaller than the interior V-groove for receiving the weldmaterial.

In conjunction with formation of the asymmetrical double V-groove weldshown in FIG. 38, the weld 2026 at the joints 2021 or 2023 is preferablymade from the outside first followed by the weld from the inside. Thisis opposite to the standard practice used in welding cylindricalvessels, which seeks to minimize welding inside confined spaces.Accordingly, the exterior weld mass 2034 is formed first followed by theinterior weld mass 2032. Therefore, this welding sequence which is theobverse of the welding sequence in FIG. 37 will minimize the tensilestress in the outer surface and the associated heat affected zone (HAZ).The interior weld may be ground flush after formation. A small convexweld crown left on the outside may be left for further mechanical workto further mitigate the onset of SCC, as further described herein.Preferably, the welding process is performed to avoid angular distortionof the workpiece. It bears noting that in all applicable embodimentsdescribe herein, the interior and exterior weld masses 2032, 2034 arejoined in the intermediate section of the joint 2021/2023 which formsthe full thickness butt weld.

Formation of the full thickness welds 2026 in FIG. 38 may be completedvia any suitable conventional welding process in a manner known in artgenerally involving the use of a non-consumable electrode or consumablewelding electrodes, wires, or rods to deposit or form weld material inthe joints 2021 or 2023 via successive passes through the weld joints.With each pass of the welding electrode or rod, the weld material is fedinto the joint and gradually built up in layers or beads to the finalprofile shown. The weld bevels shown includes a convexly shaped broadface or crown at one end and a narrow root at the base of the weldbevel. Non-limiting examples of weld processes that may be used includegas metal arc welding (GMAT), gas tungsten arc welding (GTAW), shieldmetal arc welding (SMAW), flux cored arc welding (FCAW), submerged arcwelding (SAW), and others. These welding processes are well known in theart without undue elaboration.

According to another aspect of the invention, narrow joint or groovetype welding techniques may also be used to minimize the extent of theHAZ in the base material of the shell 2028 adjacent the joint formitigating initiation of SCC. FIG. 39 depicts a full or throughthickness double sided submerged arc butt weld having a narrow profilegroove. As seen, both the interior and exterior weld masses 2032, 2034having weld bevels generally smaller than conventional double V-groovewelds as shown in FIG. 37. The lateral width of the weld between thebevels is also narrower than the conventional double V-groove weld. Thenarrow weld profile of FIG. 39 results in a reduced width HAZ andconcomitantly smaller tensile stress field which is beneficial formitigating the onset of SCC.

FIG. 40 depicts another example of a full or through thickness hybridlaser weld having a narrow profile groove formed by the hybrid laser arcwelding (HLAW)technique. Hybrid laser welding generally combines the useof a laser with GMAW or GTAW and is well known in the art without undueelaboration. This welding technique absolutely and advantageouslyminimizes the heat input imparted to the shell required to complete thebutt weld. HLAW requires less than ¼th of the heat input required by thesome non-laser welding processes noted above and produces a smallercrown on the inside (FIG. 40) or outside (FIG. 41) whose size can beadjusted by the weld wire feed rate. The result is an extremely narrowweld profile with greatly reduced HAZ in the canister shell basematerial. Forming the HLAW weld from inside of the canister shell 2028which is the obverse of standard industry practice results in nosignificant crown on the exterior surface of the shell, as shown in FIG.40. This is further advantageous for reducing the probability of SCCinitiation at the exterior surface of the shell in the HAZ.

According to another aspect of the invention, a second measure tomitigate the onset of SCC at the exterior surface of the canister shell2028 in the HAZ is full or through-thickness compaction (TTC). It willbe recalled that the welding process creates a tensile stress field inthe HAZ of the canister shell (which includes the weld mass), which isone of three conditions necessary for SCC initiation. In one embodiment,the process for fabricating a welded SNF canister 2020 comprisesmechanically working the weld and HAZ to change the stress field in theweld and HAZ region from tensile to compressive for the full thickness Tof the material using TTC. This process is referred to herein as“through-thickness compaction (TTC).” It is important to note that TTCcontrasts to and is different from the effect of surface peening, whichis only capable of inducing a compressive in-plane stress field in a 2to 4 mm depth of the shell base material from the exterior surface 2024of the canister shell 2028, but leaves a corresponding tensile stressfield in the layer below to the interior surface 2022. Advantageously,the TTC process changes the stress field to compressive from theexterior surface 2024 of the shell 2028 to the interior surface 2022 foran added level of protection against SCC.

The TTC method or process generally comprises two steps or stages.First, shape the outer surface of the weld crown during the weldformation process, as guided by an elastic-plastic finite elementanalysis explained below to the optimal size. The extent of requiredcrown is quite small; it should be based on the extent of flattening viaTTC needed to induce at least 5%, but no more than 10%, maximumcompressive strain in the weld mass and HAZ after TTC. Second, the use amechanical compression or compaction process to flatten the crownrendering the top surface of the HAZ flush or coplanar with the adjacentexterior surface 2024 of the shell 2028. The term coplanar means thatfor a flat sheet or sheet of shell material, the crown would be lie inthe same flat plane as the portions of the sheet(s) adjoining the weld.For a rolled or contoured shell, the crown would lie in the samecylindrical plane as the sheet(s) adjoining the weld.

Cold rolling or flattening the crown and adjoining HAZ by applying atargeted compressive force on it (see, e.g. FIG. 42), as the classictheory of plasticity teaches, will produce a compressive in-plane stressfield in the weld mass and its adjacent heat affected zone. Thecompaction force F should preferably be of sufficient magnitude toconvert the residual tensile stress field resulting from welding to acompressive stress for the full thickness T of the canister shell 2028.Accordingly, the in-plane stress field will be compressive across theentire thickness of the welded region (i.e. weld and HAZ) for theinterior to exterior surface 2022, 2024.

Mechanically working the weld and HAZ via TTC can be performed using avariety of commercially available processes and machines. As oneexample, the compaction device may be a suitably sized “rolling mill”using rollers as a compaction member 2042 to apply the necessarycompaction or compressing pressure or force F to the canister shell basematerial in the HAZ and weld for in-plane flattening. The interior sideof the weld 2026 and HAZ inside the canister shell 2028 is supported viaa rear support member 2040, which may be flat steel plate or other typestructure in some embodiments. The weld and HAZ of the canister shell iscompressed or compacted between the back support member 2040 and thecompaction member 2042 in a squeezing type action. In other embodiments,TTC flattening may be performed by a suitable forging technique. Forexample, a hammer forge type machine may be used as the compactiondevice which comprises an axially reciprocating die or hammer for thecompaction member 2042 to shape the weld crown by creating localizedcompressive forces in the weld and adjoining HAZ material. The hammermay be pneumatically driven to repetitiously strike the crown of theweld 2026 and adjoining HAZ with sufficient compaction force F for apredetermined period of time to flatten the weld crown in-plane with thecanister shell outer surface 2024 and concomitantly create a full depthcompressive stress field in the HAZ and weld. It is well within theambit of those skilled in the art to select an appropriate mechanicalcompaction or compression machine and process parameters to achieve theforegoing desired results and end product weldment. It will beappreciated that required force F for TTC cannot be delivered via manualmeans (e.g. manually wielded hammers or similar methods).

The shell weldment resulting from TTC will exhibit a compressive stressfield for the full thickness T of the shell in the weld and HAZ. Such astress field condition could be verified by SEM (scanning electronmicroscope) or other metallurgical examination techniques used in theart. These examination techniques are capable of generally identifyingthe microstructure of the weld and HAZ material to disclose informationabout the types of processes used on the material during fabricationsuch as TTC.

Following the TTC treatment of the weld 2026 and adjoining HAZ of theshell base material, the top surface of the weld crown is flattened andrendered flush or coplanar with the adjacent portions of the exteriorsurface 2024 of the canister shell 2028. Notably, the top surface of theweld is coplanar with the exterior surface of the shell without the useof mechanical grinding, which is a technique commonly used in industryto eliminate the crowns of welds resulting in a flush outer surface. Insome embodiment, the thickness T of the weld base material may actuallybe slightly smaller or thinner in thickness T at weld and HAZ locationthan adjacent portions of the shell 2028 due to the TTC process.

It bears noting that the TTC process may be performed when the shellsheet or sheets are either in a flat condition or a curved/contouredcondition after rolling and bending. The former might be used if a shellsegment includes more than one longitudinal weld joint.

Combining peening the welded region with TTC explained above after TTCcan be used to further increase the compressive stresses in the toplayer making the shell's exterior surface armor against salt air attackeven stronger. The alleged drawback of surface peening, namely leavingan undesirable tensile stress field in the shell layers below 2-4 mm indepth will also be ameliorated because of the compressive pre-stressgenerated by the full “through-thickness compaction” before peeningaccording to the TTC process disclosed herein.

FIG. 43 is a process flow chart summarizing the foregoing steps infabricating a welded nuclear waste canister which is resistant to theonset of stress corrosion cracking (SCC). The process or method 2050starts in step 2052 by providing a stainless steel shell 2026 having anopen longitudinal joint 2021 or circumferential joint 2023 with shellweld end or edge 2030 preparations selected commensurate for the type ofweld to be made. In one preferred but non-limiting embodiment, hybridlaser welding and end preparations may be used. In step 2054, the weldis formed as previously describe herein. In step 2056, through-thicknesscompaction (TTC) is performed. In a final optional step 2058, surfacepeening may be performed to add an additional layer of protectionagainst SCC at the outer surface 2024 of the shell 2028 in the weld andHAZ.

IV. Inventive Concept 4

With reference to FIGS. 44-49, a fourth inventive concept will bedescribed.

The process or method to protect the shell-type weldments from stresscorrosion cracking (SCC) due to prolonged exposure to the halide bearingaqueous ambient environment according to the present disclosure is nowfurther described. FIGS. 44-45 and 47-48 depict a cylinder shell-typewelded assembly or “weldment” which may be formed using theSCC-inhibiting method disclosed herein.

Referring now to FIGS. 44-45 and 47-48, a shell weldment 3020 comprisesa shell 3021 having a longitudinal axis LA which may be formed of one ormore stacked and abutted welded shell segments 3022. In the non-limitingexample shown, the shell 3021 may comprise a first segment 3022-1 andsecond segment 3022-2. Each segment is formed of an originally flatmetal workpiece comprising a plate that has been rolled by aconventional mechanical rolling process into the cylindrical shapeshown. Such mechanical rolling fabrication processes are well known inart without undue elaboration here. As a result of rolling formation,each segment 3022-1, 3022-2 comprises a single open longitudinal buttjoint or seam 3024, which is then welded to structurally join theopposing adjacent paired longitudinal edges 3030 along the weld joint orseam together. This closes the seam and ultimately contributes toforming a hermetically sealed internal cavity 3023 of the shell.

Shell segments 3022-1 and 3022-2 may be abutted in stacked end 3031 toend 3031 relationship to form a circumferential butt joint or seam 3025,which is then welded to form a weld 3029 and structurally join the twoshell segments together, thereby collectively creating the entire shellweldment 3020. The circumferential welded seam 3025 is orientedperpendicularly to longitudinal axis LA. The circumferential welded seam3025 between the segments 3022-1, 3022-2 may follow a straight arcuateand circular path (versus undulating) such that the ends 3031 of theshell segments share a common reference plane. The longitudinal weldedseams 3024 may be linear and parallel to longitudinal axis LA as shownin FIG. 44.

The circumferential and longitudinal welds 3029 may each be fullpenetration or thickness welds having a transverse configuration orprofile as shown in FIG. 47. This figure shows a transverse crosssection through one of the welded longitudinal seams 3024, but isrepresentative of the same transverse profile of the weldedcircumferential seam 3025 between the segments 3022-1, 3022-2. Each weld3029 extends for the full thickness T1 of the shell segment basematerial from an interior surface 3026 to an exterior surface 3027 ofthe shell(s).

In one embodiment, the weld may be a double-V groove weld having thedouble-sided V-groove and weld profile shown in FIG. 47. This figureshows conventional double-V groove weld end preparations of the shellworkpiece material for forming double-V welds having a correspondingdouble-V joint profile. Double-V groove welds typically require lessweld filler material are generally preferred when the weld joint isaccessible for welding from both sides of the shell (i.e. inside andoutside) in contrast to a single-V joint welds. In addition, double-Vwelds may have concomitantly reduced residual stresses and longitudinalshrinkage; the former of which is advantageous to minimize exposure toSCC. Other type welds and weld profiles, however, may be used in othersituations and embodiments.

In one embodiment, each shell segment 3022-1, 3022-2 may preferably beformed of a corrosion resistant metal particularly when used to form aspent nuclear fuel storage canister. The metal preferably may bestainless steel, and more preferably an austenitic stainless steel inone non-limiting example. The shell segments may have any suitablethickness T1 depending on the structural requirements for the vessel.

It bears noting that each longitudinal or circumferential weld isactually formed by multiple “passes” or “runs” by the welder (manual orautomated welding machine); each of which deposits weld metal (i.e. weldbead) into the double-V groove joint to successively build the weld tothe final shape and configuration shown in FIG. 47. The root pass or runis the first run or pass on the still open joint to deposit the initialweld bead in the inner-most portion of the groove. The root passprovides a base for subsequent filler passes to radially build out thecomplete double-V weld. Accordingly, each weld 3029 is collectivelyformed of a plurality of weld beads of weld filler metal, which is awell known fact and concept in the art.

The completed shell weldment 3020 may further comprise an end plate 3035on each end of the shell 3021 to completely enclose the internal cavity3023 once the spent nuclear fuel has been emplaced in the shell. The topend plate 3035 may be a final cover plate sealed after emplacement offuel in the shell canister. Each end plate 3035 may be welded to theshell via a suitable weld, which may be a fillet type weld in oneembodiment or other. These end plate welds may optionally be subjectedto the same peening operations described herein for the circumferentialweld seam 3025 between the two shell segments 3022-1, 3022-2 andlongitudinal weld seams 3024 of each segment, as further describedherein.

The most vulnerable region in the shell weldment to SCC is the weld lineand the contiguous adjoining metal mass of the shell known as the “heataffected zone” or HAZ, collectively referred to herein as the “SSCsusceptible strip” or “SSS”, or alternatively simply the weld zone. Thefabrication method or process for forming the shell weldment 3020presented herein to increase the resistance of this ambient-exposed SSSto SCC generally comprises of a judicious use threemanufacturing/fabrication operations performed in proper sequence in oneembodiment; namely: (1) Workpiece Rolling; (2) Welding; (3) Hard RollingWeldment; and (4) Peening. Each operation is further described below insequence.

Rolling: The manufacturing of the shell weldment 3020 begins withmechanically rolling the stainless steel workpiece plate to form thecylindrical shape of the first or second shell segment 3022-1, 3022-2.Preferably, cold rolling may be used if possible to provide exactingdimensions of the segments versus hot rolling. Rolling, as noted above,produces a compressive surface stress at the exterior surface 3027 ofthe shell 3021, which is an antidote to SCC. When the workpiece platestock is first rolled into the cylinder, the entire external surface ofthe curved shell develops a compressive stress field that protects itagainst SCC.

Welding: Welding, in contrast to rolling, generally produces tensilestress in the SSS across the entire thickness of the weld mass due toweld shrinkage, with the highest values reached at the outer or exteriorsurfaces 3027 of the shell most susceptible to SSC given the properambient conditions. Unfortunately, this counteracts the benefit ofinitially shaping the shell 3021 by rolling as described above. Whilethe tensile stress produced by welding cannot be entirely eliminated, itcan be significantly mitigated by utilizing the bevel detail presentedin FIG. 47 (or similar), carefully controlling the heat input, andpreferably welding the outside/exterior weld 3029 bevel first proximateto exterior surface 3027, then followed by welding the inside weld 3029bevel proximate to interior surface 3026 of the shell. Accordingly, theexterior weld bevel may be completely formed prior to forming theinterior weld bevel of the double-V butt weld to decrease the tensilestress field on the exterior of the shell 3021 at the weld lines. Thisapproach is contrary to the conventional wisdom and preferred manner inthe art of forming double-V welds, which is to gradually build the weldradially outward from the deepest part or root of the weld jointuniformly from each side to prevent distortion of the workpiece. Theconventional approach, however, is not beneficial for preventing SCCsince creation of tensile stress on the outer or exterior surface of ashell most susceptible to SCC is not minimized to the greatest extentpossible.

Any suitable welding process may be used for welding the longitudinaland circumferential seams. One non-limiting example of a suitable methodis the submerged arc welding (SAW) process. Other welding processes maybe used however to form the double-V groove butt welds in otherinstances such as without limitation shielded metal arc welding (SMAW),gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), orothers. The welding process used is not limiting of the invention.

Additional treatment, however, is still preferably desired to render thesurface stresses compressive in the SSS (“SSC susceptible strip”) afterwelding, as now further described below.

Hard Rolling of the Cylindrical Weldment: One advantageous ameliorativestep to mitigate SSC is to next mechanically hard roll (i.e. cold roll)the entire shell 3021 again including along the SSS (i.e. weld line andadjoining HAZs) after welding the shell weldment 3020. In someembodiments, at least the SSS is hard rolled if not the entire shell.Hard rolling is a process of applying high compressive surface pressurevia a mechanical roller such that the nominal contact stress at theroller-to-shell interface is in the plastic range of the base shellmaterial. This has the effect of inducing a superficial compressivestress on the rolled shell surfaces at the SSS.

Peening: Surface peening P1 illustrated in FIG. 48 is preferably nextapplied after the foregoing initial rolling, welding, and second hardrolling operations have been completed. Peening is, in essence, acontrolled bombardment of a metal surface with micro-impingements todevelop a compressive state of stress on the surface and in itsimmediate vicinity. Several techniques of micro-impinging a targetsurface are in use in the industry, including mechanical, hydrodynamic,and laser peening methods. While the actual method of energy delivery tothe target surface by each peening method varies, the underlyingmechanics is the same. The concentrated localized force on the impactedor peened surface causes it to deform, thereby creating a compressiveplanar stress field due to the classical Poisson effect. Theeffectiveness of the method relies on the manner in which the Poissonstresses from successive local depressions interact with each other. Theoverlap between the successive impinged depressions, the magnitude ofthe impact energy, and the number of peening passes are criticalparameters that are crucial in determining the depth and planaruniformity of the compressive stress field developed. Computer CodeLS-DYNA has been invaluable in helping identify the above parameters toyield best results.

The most important requirement demanded of the peening process is thatit will impart a deep layer of compressive stress in the most vulnerableregion which is the welded region of the shell 3021 where the weld andadjoining HAZ lies. This can be achieved by repeating the peening on thewelded region in multiple passes. However, it bears noting that peeningor hammering the weld strip (weld and HAZ) has the perverse effect ofgenerating a tensile stress on the surface of the shell adjacent to thepeened area (also observed in LS-DYNA simulations).

To deal with this problem, it is proposed to successively andprogressively widen the peening strip or region using subsequent passes,as shown in FIG. 48, such that the outer edges of peened region 3040(where tensile stress will develop in the shell) is progressively pushedfarther outwards and away from the SSS (SSC susceptible strip). Becausethe peening process targets the SSS associated with only the weld seams3024, 3025, the peened region 3040 may resemble a somewhat narrow stripor band in shape confined to the general vicinity of the longitudinaland circumferential shell welds. If the multi-pass peening is adroitlyapplied (guided by LS-DYNA simulations), then it is possible to move thelocation of tensile stress well outside the SSS, and in the case of thecircumferential weld seam 3025, make its magnitude small enough to beovershadowed by the compressive stress installed or imparted to theshell by the hard rolling operation performed immediately before thepeening operation. In the longitudinal weld seam 3024 direction, theresidual compressive stress created by hard rolling is less significant;therefore, the control of the heat input during welding and use ofprogressively wider peening strips 3040-1, 3040-2, 3040-3, and 3040-4 asillustrated are even more critical to the prevention of SCC.

It bears noting that in FIG. 48, the first peening pass is configuredand designed to create the narrowest peened region 3040-1 in the SSSregion. Subsequent peening passes are each selected to createprogressively widen the peened regions 3040-2 to 3040-4, with the lattercreating the widest peened region 3040-4 having edges at the interfacebetween un-peened and peened areas of the shell (where the tensilestress will develop) located sufficiently distal to the weld and HAZ toprevent the onset of SCC associated at the weld seams.

In the illustrated embodiment, four peening passes are discloses whichis not limiting of the number of peening passes that be used to createthe peened regions 3040 along the weld lines. Other embodiments may thususe more or less peening passes.

The beneficial end result of the peening process is that the SSS mostsusceptible to the onset of SCC has now been converted from a tensile toa compressive stress field which resists SSC, while the portions of theshell base material proximate and adjacent to the final peened strip orregion 3040-4 far removed from the SSS has a relatively narrow residualtensile stress field.

Thus, in summary, multi-step rolling in conjunction with multi steppeening operations as presented herein advantageously ensures that theSSS (SSC susceptible strip comprising the weld and its adjoining HAZ)and the remainder of the shell is devoid of residual tensile surfacestresses.

In one non-limiting example of a shell weldment 3020 which may form aspent nuclear fuel canister, the thickness T1 of the shell 3021 maytypically be about ½ to ⅝ inches. The weld bevels used on the interiorand exterior of the double-V welds 3029 may be about 37.5 degrees(nominal). The weld bevels may have a depth of about ⅛ to 3/16 inches.Other weld bevel angles, bevel dimensions, and shell thicknesses T1 maybe used and is not limiting of the invention.

FIG. 49 is a flow chart summarizing the steps of the foregoing method orprocess of forming a shell weldment 3020. This figure assumes, as anon-limiting example, that the weldment is formed by twolongitudinally-abutted shell segments 3022-1, 3022-2 each comprising alongitudinal welded seam 3024. Other examples may include a third ormore shell segments, or optionally may only include a single shellsegment depending on the length of the vessel required and size-basedfabrication limitations of the particular mechanical rolling machinesused for initially forming the cylindrical shell shapes from flat platestock.

The first step is providing the first shell plate in the form of a flatplate stock of metal such as austenitic stainless steel. Next, the flatplate is rolled into a cylindrical shape which creates an openlongitudinal butt seam along the abutted side or lateral edges of theplate. If not already having the desired double-V weld edge preparation,such an edge preparation is formed creating the double-V joint profileshown in FIG. 47. Next, a double-V weld is formed in the seam bypreferably forming the exterior weld bevel first completely, followingby forming the interior weld bevel completely. This reduces the residualtensile stresses creating by welding at the exterior surface 3027 of theweld most susceptible to SCC. The first welded shell segment 3022-1 isnow created.

Next, a similarly formed second cylindrical welded shell segment 3022-2is axially butted end-to-end to first welded shell segment 3022-1 if asecond segment is required. This creates a circumferential butt seambetween the two end of the shell segments 3022-1, 3022-2. If not alreadyhaving the desired double-V weld end preparation, such an endpreparation is formed creating the double-V joint profile shown in FIG.47. Welding the circumferential butt seam with double-V weld to closeseam is the next step to join the two shell segments together.

Now that all welds (longitudinal and circumferential) have been formed,the process continues with hard rolling entire shell including alllongitudinal and circumferential welds and their respective HAZs. Thispartially converts the residual tensile stresses created by weldingparticularly at the exterior surface of the shell 3021 into compressivestresses less susceptible to the onset of SCC. Next, each weld andassociated HAZ are successively peened in multiple passes withincreasingly and progressively wider peened regions 3040-1, 3040-2,3040-3, and 3040-4. This advantageously further increases thecompressive stress field on the exterior surface 3027 of the shell 3021at the welds 3029 and HAZs (i.e. SSS region or weld zones), in additionto moving any residual tensile stresses in the shell adjacent to thefinal peened region 3040-4 farther away from the weld lines.

The end result of the foregoing shell weldment fabrication process is avessel with residual substantially compressive stress fields at itsexterior surface 3027, particularly along the weld lines and HAZs (i.e.SSS). Such a vessel fabricated in this manner therefore lacks the neededcondition of an exterior tensile stress field along the weld lines,which is one of the required conditions for SCC to initiate in the SSS.

It bears noting that the initial shell rolling, welding, hard rolling,and peening operations of the foregoing shell fabrication process arepreferably performed in the sequence described above to optimize thecreation of residual compressive stresses in the shell weldment 3020 forSCC resistance.

While the foregoing description and drawings represent some examplesystems, it will be understood that various additions, modifications andsubstitutions may be made therein without departing from the spirit andscope and range of equivalents of the accompanying claims. Inparticular, it will be clear to those skilled in the art that thepresent invention may be embodied in other forms, structures,arrangements, proportions, sizes, and with other elements, materials,and components, without departing from the spirit or essentialcharacteristics thereof. In addition, numerous variations in themethods/processes described herein may be made. One skilled in the artwill further appreciate that the invention may be used with manymodifications of structure, arrangement, proportions, sizes, materials,and components and otherwise, used in the practice of the invention,which are particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being defined by the appended claims andequivalents thereof, and not limited to the foregoing description orembodiments. Rather, the appended claims should be construed broadly, toinclude other variants and embodiments of the invention, which may bemade by those skilled in the art without departing from the scope andrange of equivalents of the invention.

What is claimed is:
 1. A double-walled canister system for dry storageof nuclear waste material, the canister system comprising: a canistercomprising: a tubular inner shell defining an internal cavity forstoring nuclear waste material and having a first height; a first lidsealably welded to a first end of the inner shell; a primary base platedefining a peripheral edge portion and having an upturned annularclosure flange disposed on the peripheral edge portion; an annular fullthrough-wall thickness butt weld formed at an abutment joint between theannular closure flange and a second end of the inner shell whichsealably attaches the primary base plate to the inner shell; the innershell, first lid, and first end closure collectively defining ahermetically sealed primary pressure retention barrier; a tubular outershell adjoining the inner shell, the outer shell having a second heightwhich is at least coextensive with the first height of the inner shell;wherein the outer shell is welded to the canister to form a hermeticallysealed secondary pressure retention barrier; wherein a first end of theouter shell is seal welded to the first end of the inner shell and asecond end of the outer shell is seal welded to the primary base plateof the canister.
 2. The canister system according to claim 1, whereinthe canister is vertically oriented.
 3. The canister system according toclaim 1, wherein the canister and outer shell are disposed in aventilated storage cask comprising cooling air inlets disposed at afirst end of the cask, cooling air outlets disposed at a second end ofthe cask, and an annular gap formed between the outer shell and the caskin fluid communication with the cooling air inlets and outlets.
 4. Thecanister system according to claim 1, further comprising a secondarybase plate sealably welded to a first end of the outer shell, thesecondary base plate covering the primary base plate and forming ahermetically sealed third pressure retention boundary.
 5. The canistersystem according to claim 4, further comprising a second lid sealablywelded to a second end of the outer shell, the second lid covering thefirst lid and forming a hermetically sealed fourth pressure retentionboundary.
 6. The canister system according to claim 4, furthercomprising a plurality of stay fasteners attaching the second lid to thefirst lid.
 7. A double-walled canister system for dry storage of nuclearwaste material, the canister system comprising: a canister comprising: atubular inner shell defining an internal cavity for storing nuclearwaste material and having a first height; a first lid sealably welded toa first end of the inner shell; a primary base plate defining aperipheral edge portion and having an upturned annular closure flangedisposed on the peripheral edge portion; an annular full through-wallthickness butt weld formed at an abutment joint between the annularclosure flange and a second end of the inner shell which sealablyattaches the primary base plate to the inner shell; the inner shell,first lid, and primary base plate collectively defining a hermeticallysealed primary pressure retention barrier; a tubular outer shelladjoining the inner shell; wherein the outer shell is welded to thecanister to form a hermetically sealed secondary pressure retentionbarrier; wherein a skirt of the outer shell is welded to the second endof the inner shell and the primary base plate by a plurality ofcircumferentially spaced apart plug welds.